|When you are already connected to the remote network via VPN, you can access another PC via RDP because you're in the same network.|
You don't have to open a port at the router for that.
The reason, why you can't access the remote router is,
that your local router has the same ip address as the remote router.
So your PC searches this ip address locally not remotely.
So if you configure your local router, to use 192.168.0.2 (if possible and free), you should also be able to connect to the remote router via VPN.
Please send a reply, if you solved the problem !!!