I was wondering if there is a specific reason why you are not pushing your LAN network to the other side of the tunnel?
To set up a port forwarding rule, you will need to choose which ports to forward. In your situation, the OpenVPN zone will be the source zone, and the LAN zone will be the internal zone. To set up the rule, select the external ports as '50000-50050' (with the hyphen between them). This will ensure that the port forwarding only occurs when you connect to those specific ports. Other ports, such as port 80 used by the WebUI of the router, will not be forwarded, and you will still be able to access your RUT device.