, you can try delete the existing "Tennis Elbow" entries in your firewall settings. It should ask you to authorize TE again next time you launch the game & enter Network mode.
Although, I guess it won't solve your problem, as you can already connect to other people.
So it's more likely related to your router. There, only your router documentation will help you.
There's no a identical step-by-step way to solve network problem, coz each PC has its own configuration... So there's not much I can do without being physically near your PC (especially with people like Stajus
who even don't specify what Firewall they are using...
) And even so, I'm not sure I could help much. As far as I recall, I had big problem to set my own router to work with TE, and I'm even not sure I succeeded...
I may have used the "Forward all ports" option in that time, but I'm not sure. Anyway, since a while, I set my router to work in Bridge mode instead of Router mode, and the connection test is working fine...
Else, you can also try to use port 80 instead of 4321, sometimes, it helps.
And here you'll find more info about this topic :search.php?keywords=router