To be honest, I'm not quite sure where the problem is so I'll just state the facts. From your point of view, there are numerous possibilities. First of all, we have forward and backward UDP traffic. This traffic then passes through two firewalls - your own and that of the other player(s). And finally, we have two roles that your jj2 can take - server or client.
This gives us a total of 8 possibilities where things can go wrong. Luckily, only half of these apply for you, the other half apply for the guy on the other side of the chessboard. For now I'll just assume that everything works fine for the black player and focus on the white side of the chessboard (your part). From what you've told us so far, I've been able to visualize the following picture:

• You're having 30s CTO's on every server. This is caused by the UDP traffic not making its way from your client to the server and/or vice-versa.
• When playing as client, you can see others, but they can't see you. This confirms the UDP traffic from the server is reaching your client, hence you can see other players. Unfortunately your own UDP traffic fails to reach the server and that's why they cannot see you.
• When you hosted, you could see the other player and he could see you. This means the UDP traffic works properly in both ways.

Results:
server, incoming: YES When you host, you can see other players.
server, outgoing: YES When you host, others can see you.
client, incoming: YES When you join, you can see other players.
client, outgoing: NO! When you join, others cannot see you.

From the looks of it I'd say it's possible that some of the people you were talking about were having similar problems on their own. The fact that you're having CTO's on every possible server rules out one possibility. This means the problem likely resides on your side, but at this point I can only speculate what it might be.
If valmango can join your server (from the internet) at any time and there aren't any CTO's then there's an excellent chance that your UDP works properly while hosting. Note that jj2 uses predefined ports when hosting and random ports when joining. It is possible that this random port UDP traffic gets dropped somehow before it reaches the server.