Thats not so much about port forwarding and more about firewall rules. I got something similar'ish to work on Linux using openvpn with nordvpn. The way I set that up is reject every outgoing connection, except for 1194 for the vpn connection and that way you can ensure stuff goes threw the vpn. Im sure the nordvpn app also does that. So you will need to allow the internal ip of your jriverbox somehow or the outgoing package will be blocked.
If thats not possible with the app, you can use openvpn client on windows and mac too. But its gonna be more likely that packages are leaked outside the vpn connection without proper firewall setup and proper firewall setup will mean you have to undo your firewall setup when you dont want to use the vpn. (Thats what the app does for you too, it just does in in a way you dont have to think about these things, since just turning it on results in that happening)
But without any special config the openvpn client should at least still let you connect to pcs in your local network.