GR problems

So me and a bunch of friends of mine have been playing AoE through GR for a couple months now. Recently, we added a new person to our group. When he tried to start up GR it told him he had a symmetric router and it didn't go further. He did some looking on his own and found out one way to fix that is to enable UPnP whatever that is. He did that it's still giving him that problem. I was hoping one of y'all could suggest another fix which will actually work