I have this problem on the server of my team. And never the server receive something from my computer.
The port 2300 to 2305 are opened.
I did DMZ too.
I see with my internet service provider and it's not a network problem.
I formated and install the game again I always have the problem.
I have nothing which help me to know why sometimes I can join a server and sometimes I can't.
I have this problem since I have bought the game on March the 5th.