Also, and this is fucking absurd,
according to gamefaqs these are the ports you need open to play this game multiplayer:
The complete ports to use to be able to play (join) and host (host) a working joinable online content at least are as follows:
TcP: 7777, 28900, 28910, 29901, 29900, 28902, 3783,
UdP: 27900, 6500-6515, 7787, 7777-7783, 13139, 6667
Also, at least until they fix it, it may be necessary to start / host a public game, invite a friend to play in it, then end the public game (just kill it), and then start a private game to play a private multiplayer game. That's unconfirmed but Mark 'n I may have seen that behavior...