Can't connect 2PC with same IP

Hey guys ! It's Smurfed here (a grot's brother). I got a problem.

When I'm trying to join GTB server while my brother's playing i got an error... i mean it's not an error exactly but i cant join server. Just got this:

Awaiting connection 1, 2, 3, 4, 5, 6... 24, 99... ... ...

we tried to play together on 4 PC but it wasnt workin so i reasume it's a "same IP" problem.
(it's impossible for me to change IP and dont wanna use proxy)

Is there anyone who knows how to repair this issue.
Thanks in advance. Smurfed


EDIT:

When i trying to connect, in my brother's console appears this:

unwanted challenge response received ! Ignored.

EDIT2:

I found that this could be my router problem. I got a 'Linksys WRT54GCv3'... found already new firmware and will try it later.

You can have 3 pcs from the same I.P on our server.