If the auto-scan option does not display your PLC, the IP can be added manually. Simply specify the PLC LAN IP address, with the netmask of 255.255.255.255. This will add a route to a single host.
If you need to reach more than one LAN device, you can specify the IP address of the LAN network (e.g. 192.168.1.0) and the netmask in use (by default - 255.255.255.0).
More information about VPN Hub configuration can be found here.