All right, I don't know what to do now... I've opened ports 10052, 10053 and 10054, both TCP and UDP, on my Belkin wireless router (They're called Special Application Ports) and I excluded ports 10052, 10053, and 10054 from Windows Firewall, but I still get connection time out errors.

I don't know, is there anything more I can do?

I get them all the time and I don't know what causes it =D

I know what does it. If you watch logs when you play JJ2 on net, net blocks random ports from 10052 (or -53 or -54) to your computer, ports something like 1900-1999. Im not sure, so DO NOT go and open these ports, they wont help anything. Seems only good way to play JJ2 is to go and unpluck your router when you play jazz. I couldnt play JJ2 untill I unplucked my router.

Well that's also a bit of a problem because the computer I want to play Jazz on is connected to the network via the router.

JJ2 WILL work behind properly configured routers. You're likely overlooking a small detail, I spent months thinking my router was broken before I realized that the JJ2 ports that I opened weren't "active."

I do feel like I'm overlooking something simple. Could you remind me the ports you're supposed to forward?

You also might not be overlooking something simple - it might just not work. I am a perfect example.