Step 1: Set up VPN
Follow directions of VPN software to create a LAN connection. Windows PC acting as RCForb Server should run the VPN server (or a thrid Windows PC on the same network.) Server and Radio PC's should be able to "ping" each other with VPN activeactive.
I Recommend SoftEther VPN, not for its ease of setup, but for how few steps are required for setup. Both PC's, one on the home LAN and one on the VPN, should both have an IP address on the home LAN, and SoftEther has the most straightforward setup to do this.
Test functionality of VPN by moving radio PC to hotspot/public wifi not on your home network. Connect to the VPN again and you should still be able to ping the Server PC