you will need to adjust the routes to send all of the traffic from the RTU through the VPN. When the SIM is inserted the routes are adjusted as the SIM provides authorization for internet access via LTE.
If you are trying to provide a static ip address to the RTU via a VPN this may not work due to a ton of networking challenges related to interface mapping. You may want to check with a network professional regarding virtual interface bridging over OpenVPN. This is a rather complex topic that would require some work on the "PC" that is acting as the head end.