@astraljenga, the Game Client by default uses 7777, 7778 and 27015, so what matters is that your server does not use that, so for example you could use 7779, 7780 and 27016 for your server.
In this case you would have to port forward 7779, 7780 and 27016 on your router (I would suggest to do it for all protocols, not just UDP), and when that’s done you need to make sure the TEST PORT returns green.
@stinger, do you think it would make sense for me to change the default values of the ports to not be the same as what the client normally use?