Hmm, just in case this is relevant, the game server was down for possibly a couple of hours a few days ago. There was a brief power outage that reset the server, and the game server was not set to auto restart after a reboot.
There are some routers that you basically just can't program around in any reliable way. For those, if you have access, you can manually setup port forwarding (47800 UDP), and add the ForcedPort=47800 to the NetFix section in Outpost2.ini. (You can of course change the port number, as long as they match, but I recommend that default, as you won't need to specify it when doing a direct IP connect).
The problem is that the introducer (either the host or the game server) needs to introduce all the other players, and it can't introduce someone if their router maps a different port back for each IP address it sends data to. If a client sends data to the introducer from the same port as it sends to other clients, then the introducer can inform the other clients of the proper port number. For some routers, this isn't the case, and so no proper introduction is possible. The work around is to force a known port to be used, but that requires manually setting up port fowarding.
Note that port forwarding is not required by the host if all the routers are well behaved.