I have been having the same problem since the Dec. 4th PC patch. I was able to play with no problems with a moderate NAT type for weeks before Dec. 4th.
Since then I have port forwarded the correct ports, enabled UPnP, tried countless restarts (PC and router) but nothing works. Even after I opened the correct ports and enabled UPnP I'm still at a moderate NAT for some reason as well. I just want to play the game.
MP, Blackout, and Zombies will not find a match.