You need to find out exactly what the other router is doing that your Mikrotik is not doing, and then we can help figure out how to make the Mikrotik do the same.
The ultimate thing to know is: what happens on the wire?
Is any kind of tunnel being established?
Does the working router send/receive packets using the public IP address directly or does it tunnel or what?
Does the CGNat IP appear on tagged/untagged ethernet frames? What about the public IP?