IP-Forwarding works locally only although I can access my Server by public IP

[DrRiisTab]

Hello

Since 2 days I'm trying to set up a game server, where my friends should can connect to.

I installed Ubuntu server on my old computer and installed the game server. I can join the server by the local IP address and after some settings on the modem, even by the public IP address. But my friends can't.

To see what's the problem I made it as simple as possible and set up a telnet server:

• 
  - For testing purposes I deactivated the firewall on the game server
  - localIP is static
  - IP-Forwarding on modem redirects publicIP:23 to localIP:23
  - Firewall is deactivated on modem
  - telnet localIP 23 works
  - telnet publicIP 23 works
  - telnet using publicIP and Port 23 from outside (for example on http://www.adminkit.net/telnet.aspx) doesn't work

I think the problem is somewhere on the modem, but I can't guess where.
Do you have any idea what to do?

 

[AqwBroders]

Some modem's still goes into Local/Private mode even if the firewall is disabled.