Could you finally update the network configuration in the NC client so that the user does not have to even know about the "local port" setting? It would be a simple change in the way the UDP "sending" port gets selected.
Now it's a fixed port selected in the configuration, I guess to accomodate the routers and firewalls from the really ancient times when stateful tracking of connections wasn't present necessarily. Instead the UDP port should be a random port from the high port range, let's say 49152-65535, which is exactly how every other game that uses the UDP protocol is selecting the UDP port.
The change would simplify the configuration and would end the confusion about what the local port setting does, the users really don't have to know about it since using a random port works for everyone now.