Im using the same router, latest firmware version, never had any problems with any other game, and as far as peer to peer is concerned, it works perfectly fine in any other application.
Tried the guide at teh top of the page, no difference at all, can't connect to any custom game and have yet to be able to successfully connect toany other game.
I was really excited to play online when i got home, about 3 hours ago i started trying to join games, needless to say im not so excited anymore 