If you are wanting to RDP into a PC on another netwrok you can do it a couple of different ways, some different ways are:
You can configure a VPN then create an RDP session to point at the local hostname or IP address of the PC to which you wish to connect.
The reason you would point the RDP session at the local address of the host you wish to connect is because when you create a VPN you are effectivly connecting your external PC to the remote network.
You could also (if you have configured your portforwarding to do so) create an RDP session pointing at the public IP or hostname address then append the address with a for example mydomain.com:3389
This way your RDP session will be forwarded to the appropiate internal address specified in the port forwarding table without needing a VPN session to be established first.
Personally I would prefer to create a VPN connection then an RDP session pointing at the internal host address, than simply connecting via RDP over the internet.