P2P Software connection problems -router

I have Tiscali ADSL 2mpbs internet connection. Everything used to work fine until I installed my Belkin ADSl modem router – 4 port + wireless. Since then I have not been able to use my p2p software properly, and sometimes online games do not work. I have tried opening up a UDP/TDP port, but this has not worked. Maybe this was dont incorrectly – what else can I do?