Please try the next workaround, after which and depending on your results, this answer may be deleted or improved. Thank you. I know this is not a forum but I provide this as an answer because of it works for me when setting up a WAN Game Server for Team Fortress 2 and it -indeed- does an excellent work.
First of all you need to install firestarter if haven't yet. Please drop
sudo apt-get install firestarter in a terminal, provide your password and wait for the install to finish.
Next, let's run firestarter with administrative privileges by dropping
gksudo firestarter in the terminal. After which we'll be asked for our user password in GUI. Then the firestarter window will appear, as shown in the next screenshot:
Let's clic the "Policy" Tab in order to add the proper ports or port lists that your game needs by right clicking on the second list (Allow service/Port/For) and choose "Add Rule" as shown in the next screenshot:
We'll see the "Add new inbound rule" in which we'll fill the data fields with the proper values in order to allow the port's calls hit the computer's ports.
The service's ports will be added to the list, after which you can reach your game server with no difficulty.
If you are experiencing some kind of issue, you can also temporally stop the Firewall in the top side of the "Status" Tab or via Menu "Firewall" option "Stop Firewall" (Ctrl+P also does the trick). This way you can check if your router is dropping the external petitions to your computer, after which you can solve your computer's ports issue.
Please let us know how are you managing this issue in order to modify/delete this question if doesn't work for you.