If the Airouter is receiving it's own WAN address then of course it can be port forwarded from that WAN address into the local network.
It sounds to me like you are incorrectly calling the AirRouter the bridge while it is in fact the Mikrotik that is the bridge and the Airrouter is the Router.
Is it...
Incoming Link > Mikrotik (Bridge) > AirRouter (WAN address) > LAN
or is it...
Incoming Link > Airrouter (Bridge) > Mikrotik (WAN address) > LAN
The one dialing the PPPoE isn't always necessary the router as they can often be configured to still pass the WAN address directly to the next device in the chain which is the actual routing device, still making it the bridge even though it does the dialling up.
Your port forward happens on the device that receives the WAN address directly and does the NAT to you local network.
Bridge = No NAT
Router = NAT + Masquerade (hence the port foward happens here)
We need to know:
a. Which device is doing the the PPPoE connection.
b. How / where with what port is your AirRouter connected to the Mikrotik.
c. What device has the WAN IP.