PlaneShift uses UDP port 7777.
Wrong.
PlaneShift tries to send packets to
Port 7777 of the game server (laanx.fragnetics.com); but
it is not defined which port it will open on your own computer. It will probably chose any available port with a number greater than 1023, because such port numbers are usually available for short-time communication.
NAT routers will usually open the outgoing port for incoming packets. I played PlaneShift through an AVM Fritz!Box without opening my own port 7777. I even play it well through a local network with an own Windows Server being another router after the DSL router.
If your firewall is very restrictive, you may need to allow UDP packets being sent to a distant Port 7777. But your locally used UDP port will be random in a usual range.