Hello, there is only a few different things that you can try to resolve connection problems.
1.) Disable firewall(s) / add the client to the exceptions.
2.) Enable the DMZ zone on your router to your LAN.
3.) If your router does not have DMZ, then just port forward UDP 100 - 65535 to your LAN.
4.) Try turning off your anti-virus(The only reason why I said this, is because someone's nod32 antivirus was interfering with the game in some way, and helped their connection by turning it off).
(If none of those work, then it's likely that your ISP provider blocks some UDP ports, and you would have to call them to confirm.)
~RedEyes.