Fastest way is resetting the routerboard to blank settings, then applying the required ones, turning the routerboard into a wired/wireless switch.
To do so:
1.- System > Reset Configuration: enable
Keep User Configuration if you want to keep your admin user and password, and enable
No Default Configuration, in order to start from a "blank" router configuration.
2.- Log into router using Winbox Neighbors tab (click on its Mac address field)
3.- Open a new terminal and copy & paste this config:
/interface bridge add name=bridge1
/interface wireless security-profiles
set [ find default=yes ] supplicant-identity=MikroTik
add authentication-types=wpa2-psk disable-pmkid=yes eap-methods="" mode=dynamic-keys name=secure supplicant-identity=MikroTik wpa2-pre-shared-key=YOURSSIDPASSWORD
/interface wireless
set [ find default-name=wlan2 ] band=2ghz-b/g/n disabled=no frequency=auto installation=indoor mode=ap-bridge security-profile=secure ssid=YOURSSID wireless-protocol=802.11
set [ find default-name=wlan1 ] band=5ghz-a/n/ac disabled=no frequency=auto installation=indoor mode=ap-bridge security-profile=secure ssid=YOURSSID wireless-protocol=802.11
/interface bridge port add bridge=bridge1 interface=all
/ip dhcp-client add interface=bridge1
/system identity set name=Mikrotik.AP
This will turn your mikrotik into a "wired/wireless" switch, that will get an IP (for management, upgrades, etc) from the main LAN router DHCP.
That's all required.
After this config is applied you can go to the wireless interfaces and tune them to your liking (modes, channel, width, etc) via WinBox.