Could you really detect a UDP open port? Those port detection programs need to see a response in order to detect. UDP is stateless, so no handshake is required. At this point, I believe the problem lies on your inner host.
EOL means no more security updates. So when someone hacks your router, it is on you.
Port forwarding is not a "basic feature." Router routes between 2 networks and that's the complete qualification for a device to be called a router. Doing anything extra is added amnesties.
Nonetheless, PF should work on G1100. You don't need to upgrade unless you want.