I will stick to 192.168.3.0/24 for all tests (I currently have 2 ISP networks, but I will be keeping one of them).
I’ve tried some things per your suggestions and here are my observations:
The following were executed from a PC on ether5 port of AC2 (using Winbox of course).
/interface vlan
add comment=myconf interface=bridge name=AC2-LAN vlan-id=88
add comment=myconf interface=bridge name=ISP-LAN vlan-id=3
/ip dhcp-server add address-pool=default-dhcp interface=AC2-LAN name=myconf
that last command resulted in: “No IP address on interface” so I added
/ip address add address=192.168.3.7/24 comment=myconf interface=AC2-LAN
/interface bridge port print
Flags: I - INACTIVE; H - HW-OFFLOAD
Columns: INTERFACE, BRIDGE, HW, PVID, PRIORITY, HORIZON
# INTERFACE BRIDGE HW PVID PRIORITY HORIZON
;;; defconf
0 IH ether2 bridge yes 1 0x80 none
;;; defconf
1 IH ether4 bridge yes 1 0x80 none
;;; defconf
2 H ether5 bridge yes 1 0x80 none
;;; defconf
3 wlan1 bridge 1 0x80 none
;;; defconf
4 I wlan2 bridge 1 0x80 none
;;; defconf
5 IH ether3 bridge yes 1 0x80 none
/interface bridge port set [find interface=ether2] pvid=88 comment=myconf
/interface bridge port set [find interface=ether3] frame-types=admit-only-untagged-and-priority-tagged pvid=3 comment=myconf
/interface bridge port set [find interface=ether4] frame-types=admit-only-untagged-and-priority-tagged pvid=88 comment=myconf
after this all ports (ether2, ether3, ether4, ether5) did have internet and the IP from any port was 192.168.88.254 no matter which VLAN they “belong” to.
/interface bridge port add bridge=bridge interface=ether1 pvid=3 comment=myconf
after this all ports (ether2, ether3, ether4, ether5) did have internet but the IP from any port was 192.168.3.79 (IPs tested after ipconfig /release, /renew)
/interface bridge port print
Flags: I - INACTIVE; H - HW-OFFLOAD
Columns: INTERFACE, BRIDGE, HW, PVID, PRIORITY, HORIZON
# INTERFACE BRIDGE HW PVID PRIORITY HORIZON
;;; myconf
0 IH ether2 bridge yes 88 0x80 none
;;; myconf
1 IH ether4 bridge yes 88 0x80 none
;;; defconf
2 H ether5 bridge yes 1 0x80 none
;;; defconf
3 wlan1 bridge 1 0x80 none
;;; defconf
4 I wlan2 bridge 1 0x80 none
;;; myconf
5 IH ether3 bridge yes 3 0x80 none
;;; myconf
6 H ether1 bridge yes 3 0x80 none
MikroTik was power cycled resulting in:
ether2(vlan 88): IP is 192.168.88.250, can’t ping any 192.168.3.x device except 192.168.3.7 (AC2-LAN) and 192.168.3.165 (ether1)
ether3(vlan 3) : IP is 192.168.3.79, can’t ping any 192.168.88.x device and 192.168.88.252 (wlan2) can’t ping any 192.168.3.x device except 192.168.3.7 (AC2-LAN) and 192.168.3.165 (ether1)
ether4(vlan 88): same IP and behavior as ether3! (shouldn’t this be getting 192.168.88.x IP from VLAN 88 like ether2?)
ether5(vlan 1) : same IP and behavior as ether3!
I forgot to check internet connectivity though
Re-testing
ether2(vlan 88): No internet, IP is 192.168.88.250, same as before.
ether3(vlan 3) : No internet, IP is 192.168.88.250, flipped !
ether4(vlan 88): internet OK, IP is 192.168.3.79, flipped !
ether5(vlan 1) : No internet, IP is 192.168.88.250, flipped !
It seems no matter to which port the PC is connected, it gets an IP from the ISP or from AC2 network randomly!
If IP belongs to the ISP network it has internet, otherwise it doesn’t.
/interface bridge port add bridge=bridge interface=ether1 pvid=3 comment=myconf
If this is disabled or deleted, all ports get internet no matter in which random network they get assigned to.
Also after disabling, PCs on either network can ping the other network.
I am thoroughly confused.
/interface bridge port print
Flags: X - DISABLED, I - INACTIVE; H - HW-OFFLOAD
Columns: INTERFACE, BRIDGE, HW, PVID, PRIORITY, HORIZON
# INTERFACE BRIDGE HW PVID PRIORITY HORIZON
;;; myconf
0 IH ether2 bridge yes 88 0x80 none
;;; myconf
1 IH ether4 bridge yes 88 0x80 none
;;; defconf
2 H ether5 bridge yes 1 0x80 none
;;; defconf
3 wlan1 bridge 1 0x80 none
;;; defconf
4 I wlan2 bridge 1 0x80 none
;;; myconf
5 IH ether3 bridge yes 3 0x80 none
;;; myconf
6 X ether1 bridge 3 0x80 none
I didn’t try anything else because I don’t understand what’s going on.
, my bad, ether3 is not “connected” to anything, and when you add an address to it, a new route is created that goes in ECMP with the one already created by the connection on ether1, so it is essentially mixing up LAN with WAN.
on the other thread working (on a CHR in GNS3), there were a couple typos and a missing setting (the vlan filtering enabled on the bridge, obvious, but it took me a lot of time to find it 
:
) is whether these two ports (and eher2) should have “frame-types=admit-only-untagged-and-priority-tagged” (like ether4 currently has) or not.