|Since the LAN connection works and external connections don't, that doesn't leave a lot of options.|
At this point if I were you, I'd call my ISP and make sure they're not blocking this type of traffic on this port..........it's possible they are.
If they aren't, that only leaves, a couple options:
1) The internet is broken and nobody else can reach you
2) Users trying to connect from remote locations as a client to your server are screwing something up
3) Your port forward is configured incorrectly.
#1 is easy enough to figure out. If you can cruise the internet, then it's working. #2 is also easy enough to figure out. All the client should require is your external IP address. The port number is generally not required since the game you're attempting to connect with operates on said port. That leaves only #3
Just FYI, I've setup numerous different games on my home LAN so a friend, or on occasion, a couple other friends, who live far away from me could connect to the game I was running in host (server) mode so we could all play the game(s) together.
In every instance, I had to create a port forward on my router to allow them through. I know from extensive experience, if you mess it up, nobody can remotely connect while local connections can.
What I can tell you beyond a shadow of a doubt is, if you're internet is working, and your LAN is working, this is not a networking issue
Considering it's not an actual networking problem and this is a networking forum, at this point in time if I were you, I'd find a forum that deals specifically with the game in question and join it. Chances are someone else has already had the exact same issue and with a bit of searching and some reading, you'll find a way to make yours work. If not, then you ask.
If there are no forums for your specific game, you could always email the company that created the game and ask them directly.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.