To narrow down your problem, try ruling out other variables:

Ok, you can first download the chatserver I made
http://www.wasteland.at/GSTNet_chatserver.zip
(runs on port 2300)

This server-program works, and with this you can test if your problem
is connected to your firewall/router.

If it works, you can rule out, that this is merely a simple "firewall" blocking,
or wrong router setting.
Then you know this is caused by a 3dgs problem.