Would love to see configurations of all devices (/export hide-sensitive file=anynameyoulike)
You can upgrade by placing the correct package on the device. That won't solve your DNS problem, but will upgrade the device.
Router A
# jan/27/2022 18:36:21 by RouterOS 6.49.2
# software id = 42LP-PZFZ
#
# model = 960PGS
# serial number = AD8A0AD5C431
/interface bridge
add name=bridge1
/interface ethernet
set [ find default-name=ether1 ] speed=100Mbps
set [ find default-name=ether2 ] speed=100Mbps
set [ find default-name=ether3 ] speed=100Mbps
set [ find default-name=ether4 ] speed=100Mbps
set [ find default-name=ether5 ] speed=100Mbps
set [ find default-name=sfp1 ] advertise=\
10M-half,10M-full,100M-half,100M-full,1000M-half,1000M-full disabled=yes
/interface list
add name=WAN
add name=LAN
/interface wireless security-profiles
set [ find default=yes ] supplicant-identity=MikroTik
/ip pool
add name=dhcp ranges=192.168.11.100-192.168.11.254
/ip dhcp-server
add address-pool=dhcp disabled=no interface=bridge1 lease-time=2h name=dhcp1
/user group
set full policy="local,telnet,ssh,ftp,reboot,read,write,policy,test,winbox,pas\
sword,web,sniff,sensitive,api,romon,dude,tikapp"
/interface bridge port
add bridge=bridge1 interface=ether2
add bridge=bridge1 interface=ether3
add bridge=bridge1 interface=ether4
add bridge=bridge1 interface=ether5
add bridge=bridge1 interface=sfp1
/interface bridge port-extender
# no hw support
set control-ports=ether4 switch=switch1
/ip neighbor discovery-settings
set discover-interface-list=!dynamic
/interface list member
add interface=ether1 list=WAN
add interface=bridge1 list=LAN
/ip address
add address=192.168.11.1/24 interface=ether2 network=192.168.11.0
/ip dhcp-client
add disabled=no interface=ether1
/ip dhcp-server network
add address=192.168.11.0/24 dns-server=195.39.77.66,8.8.8.8 gateway=\
192.168.11.1
/ip firewall nat
add action=masquerade chain=srcnat
/ip firewall service-port
set ftp disabled=yes
set tftp disabled=yes
set irc disabled=yes
set h323 disabled=yes
set sip disabled=yes
set pptp disabled=yes
set udplite disabled=yes
set dccp disabled=yes
set sctp disabled=yes
/ip route
add distance=1 dst-address=192.168.88.0/24 gateway=192.168.11.253
/ip service
set telnet disabled=yes
set ftp disabled=yes
set ssh disabled=yes
set api disabled=yes
set api-ssl disabled=yes
/system clock
set time-zone-name=Europe/Prague
/system identity
set name="MikroTik master"
/tool graphing resource
add
/tool sniffer
set filter-interface=ether4 only-headers=yes
Router B
# jan/27/2022 18:38:46 by RouterOS 6.48.1
# software id = V2ZC-74WQ
#
# model = RouterBOARD 962UiGS-5HacT2HnT
# serial number = 8A770946EBA1
/interface bridge
add admin-mac=B8:69:F4:36:15:D5 auto-mac=no comment=defconf name=bridge
/interface wireless
set [ find default-name=wlan1 ] band=2ghz-b/g/n channel-width=20/40mhz-XX \
country="czech republic" distance=indoors frequency=auto installation=\
indoor mode=ap-bridge ssid=Raki-Trebo-2 wireless-protocol=802.11
set [ find default-name=wlan2 ] band=5ghz-a/n/ac channel-width=\
20/40/80mhz-XXXX country="czech republic" disabled=no distance=indoors \
frequency=auto installation=indoor mode=ap-bridge ssid=Raki-Trebo-2 \
wireless-protocol=802.11
/caps-man security
add authentication-types=wpa-psk,wpa2-psk encryption=aes-ccm \
group-encryption=aes-ccm name=security
/interface list
add comment=defconf name=WAN
add comment=defconf name=LAN
/interface wireless security-profiles
set [ find default=yes ] authentication-types=wpa-psk,wpa2-psk group-ciphers=\
tkip,aes-ccm mode=dynamic-keys supplicant-identity=MikroTik \
unicast-ciphers=tkip,aes-ccm
/ip pool
add name=dhcp ranges=192.168.88.10-192.168.88.254
/ip dhcp-server
add address-pool=dhcp interface=ether2 name=defconf
/user group
set full policy="local,telnet,ssh,ftp,reboot,read,write,policy,test,winbox,pas\
sword,web,sniff,sensitive,api,romon,dude,tikapp"
/caps-man manager
set enabled=yes
/interface bridge port
add bridge=bridge comment=defconf interface=ether2
add bridge=bridge comment=defconf interface=ether3
add bridge=bridge comment=defconf interface=ether4
add bridge=bridge comment=defconf interface=ether5
add bridge=bridge comment=defconf interface=sfp1
add bridge=bridge comment=defconf interface=wlan1
add bridge=bridge comment=defconf interface=wlan2
add bridge=bridge interface=ether1
/ip neighbor discovery-settings
set discover-interface-list=LAN
/interface list member
add comment=defconf interface=ether1 list=WAN
add interface=ether2 list=LAN
add interface=ether3 list=LAN
add interface=ether4 list=LAN
add interface=ether5 list=LAN
add interface=sfp1 list=LAN
add interface=wlan2 list=LAN
add interface=wlan1 list=LAN
/interface wireless cap
set bridge=bridge interfaces=wlan2,wlan1
/ip address
add address=192.168.88.1/24 comment=defconf interface=ether2 network=\
192.168.88.0
/ip dhcp-client
add interface=bridge
/ip dhcp-server network
add address=192.168.88.0/24 comment=defconf gateway=192.168.88.1 netmask=24
/ip dns
set allow-remote-requests=yes servers=185.140.244.62,8.8.8.8
/ip dns static
add address=192.168.88.1 comment=defconf name=router.lan
/ip firewall filter
add action=accept chain=input comment=\
"defconf: accept established,related,untracked" connection-state=\
established,related,untracked
add action=drop chain=input comment="defconf: drop invalid" connection-state=\
invalid
add action=accept chain=input comment="defconf: accept ICMP" protocol=icmp
add action=accept chain=input comment=\
"defconf: accept to local loopback (for CAPsMAN)" dst-address=127.0.0.1
add action=drop chain=input comment="defconf: drop all not coming from LAN" \
in-interface-list=!LAN
add action=accept chain=forward comment="defconf: accept in ipsec policy" \
ipsec-policy=in,ipsec
add action=accept chain=forward comment="defconf: accept out ipsec policy" \
ipsec-policy=out,ipsec
add action=fasttrack-connection chain=forward comment="defconf: fasttrack" \
connection-state=established,related
add action=accept chain=forward comment=\
"defconf: accept established,related, untracked" connection-state=\
established,related,untracked
add action=drop chain=forward comment="defconf: drop invalid" \
connection-state=invalid
add action=drop chain=forward comment=\
"defconf: drop all from WAN not DSTNATed" connection-nat-state=!dstnat \
connection-state=new in-interface-list=WAN
/ip firewall nat
add action=masquerade chain=srcnat comment="defconf: masquerade" disabled=yes \
ipsec-policy=out,none out-interface-list=WAN
/ip route
add distance=1 gateway=192.168.11.1
/system identity
set name="MikroTik Wifi Obyvak"
/tool mac-server
set allowed-interface-list=LAN
/tool mac-server mac-winbox
set allowed-interface-list=LAN
Router C
# jan/27/2022 18:39:42 by RouterOS 6.49.2
# software id = W3SA-PWMP
#
# model = RB951G-2HnD
# serial number = DE350DE38DD8
/interface bridge
add admin-mac=08:55:31:EF:3A:DE auto-mac=no comment=defconf name=bridge
/interface wireless
set [ find default-name=wlan1 ] band=2ghz-b/g/n channel-width=20/40mhz-XX \
country="czech republic" disabled=no distance=indoors frequency=auto \
installation=indoor mode=ap-bridge ssid=Raki-Trebo-Wrk-2 \
wireless-protocol=802.11
/interface list
add comment=defconf name=WAN
add comment=defconf name=LAN
/interface wireless security-profiles
set [ find default=yes ] authentication-types=wpa-psk,wpa2-psk mode=\
dynamic-keys supplicant-identity=MikroTik
/ip pool
add name=dhcp ranges=192.168.99.10-192.168.99.254
/ip dhcp-server
add address-pool=dhcp disabled=no interface=bridge name=defconf
/interface bridge port
add bridge=bridge comment=defconf interface=ether2
add bridge=bridge comment=defconf interface=ether3
add bridge=bridge comment=defconf interface=ether4
add bridge=bridge comment=defconf interface=ether5
add bridge=bridge comment=defconf interface=wlan1
add bridge=bridge interface=ether1
/ip neighbor discovery-settings
set discover-interface-list=all
/interface list member
add comment=defconf interface=ether1 list=WAN
add interface=ether2 list=LAN
add interface=ether3 list=LAN
add interface=ether4 list=LAN
add interface=ether5 list=LAN
add interface=wlan1 list=LAN
/interface wireless cap
set bridge=bridge interfaces=wlan1
/ip address
add address=192.168.99.1/24 comment=defconf interface=ether2 network=\
192.168.99.0
/ip dhcp-client
# DHCP client can not run on slave interface!
add comment=defconf disabled=no interface=ether1
/ip dhcp-server network
add address=192.168.99.0/24 comment=defconf dns-server=192.168.88.1 gateway=\
192.168.99.1 netmask=24
/ip dns
set allow-remote-requests=yes servers=185.140.244.62
/ip dns static
add address=192.168.99.1 comment=defconf name=router.lan
/ip firewall filter
add action=accept chain=forward comment="defconf: accept in ipsec policy" \
ipsec-policy=in,ipsec
add action=accept chain=forward comment="defconf: accept out ipsec policy" \
ipsec-policy=out,ipsec
add action=fasttrack-connection chain=forward comment="defconf: fasttrack" \
connection-state=established,related
add action=accept chain=forward comment=\
"defconf: accept established,related, untracked" connection-state=\
established,related,untracked
add action=drop chain=forward comment="defconf: drop invalid" \
connection-state=invalid
add action=drop chain=forward comment=\
"defconf: drop all from WAN not DSTNATed" connection-nat-state=!dstnat \
connection-state=new in-interface-list=WAN
/ip firewall nat
add action=masquerade chain=srcnat comment="defconf: masquerade" disabled=yes \
ipsec-policy=out,none out-interface-list=WAN
/ip route
add distance=1 gateway=192.168.11.1
/system clock
set time-zone-name=Europe/Prague
/system identity
set name="MikroTik Wifi Pracovna"