I always chicken my worms when using productive test!
Have you tried turning OFF UPnP and just having the necessary port forward rules to the Plex Server?
If the Plex Server will only work with the router firewall off, leave the Firewall ON and bind its IP Address (the plex server) to the DMZ and try that.
That way only the Plex Server is open to all traffic and not your LAN.
