Well, forwarding those ports is what is needed. In fact, to join other games most people really only need 10052 UDP, because outgoing TCP connections are usually able to be made. For hosting, you need 10052 TCP as well (so other people can connect to your game). It is only if outgoing TCP connections are blocked that you need to open 10054 TCP (and again, this is only needed for hosting, to connect to the list server).

When you join a game, are you able to be in the server and see chat for about 30 seconds before timing out? Or are you not able to connect at all?

If the former is the case, there is a problem with the UDP forwarding. JJ2 sends server data, like the level file, tileset file, player list, chat, and people leaving\joining on 10052 TCP. That is also where the initial connection is made. But actual game data, like player movements, weapons, roasts, etc, is sent on 10052 UDP. So port 10052 UDP must be forwarded both ways, otherwise the user will time out after thirty seconds.

If the latter is the case, there is a problem with outgoing TCP connections.

Either way, this is really a problem you should discuss with your network administrator. The general solution is to forward ports 10052 and 10054, so if that doesn't work it is a local problem and not something we can easily help you with here.