Eventually, the client will actually detect the host

Please ignore the Italian setting, as it's unrelevant
it detects the host after two minutes of what looks like the game has frozen. But then nothing happens if I try to join. Instead it freezes again until the hosted game vanishes from the list.
I have two computers linked through a router. I know routers are a bit of a mess when it comes to networking, but I don't have a clue about this - port setting and the like. Anyone's help would be much appreciated.