In the case of Port forwarding, PLC works the same way as any other PC.
If you set correctly PFW in the router, and you can connect to PLC locally but not externally, than the most common mistake is wrong Gateway address in PLC.
Some networks may have multiple gateways (ex: Data & Voice). you have to set PLC to the one, you are using for port forwarding.
P.S.
Ping is useless to diagnose forwarded devices. The response comes from the router, not from the device.