GP3 Multiplayer Problem

i have a broadband connection coming through a Router/LAN. im having problems playing GP3 online. obviously the connection comes form a LAN, so I choose TCP/IP Network, but I still can't get online - it just says no sessions found, even though I know my mate has a session running.

is there a common problem with using a router/lan? - i know some games won't work through this setup.

anyone got any ideas?