Could you please share the configuration that you have? If your RUT955 and PLC are in different subnets there's no way they will communicate with each other. Is there a possibility to put the router or PLC into one or another subnet? You can do it on RUT955 by changing it's LAN IP to 172.16.11.1 in Network > Interfaces > LAN (assuming you're on latest firmware version 07.00).
Next thing to do would be pushing the routes into PLC subnet in OpenVPN so it would be accessible.
Most of the information should be found here: https://wiki.teltonika-networks.com/view/OpenVPN_configuration_examples
Please read it carefully.