A network issue

So most clients can see our network on the list, but from time to time comes a client that can't see anything. after doing research we found that this is a upnp issue with the router. well we want to remove this issue for all players so some players don't have to edit router settings to play on our server. fgunz did it. we want to know how we can do it too. Is there a fix for this? We've been searching all night

bump and an update... we tried giving the client access to our vpn running on my personal network which can see the servers but his network STILL couldn't connect