Hi! Facundo here again! I understand exactly what you need. I can do the port forwarding in the Mikrotik router, doing only that would be USD 50.
But I believe that NAT is much harder to deal with. I you need to configure FreePBX to be able to del with that. I can also do it. I'm bidding for this kind of project. To get everything working for you.
Hope we can chat again. For some reason I cannot talk to you anymore.
Cheers,
F.