**Update**
I visited site today as the Teltonika failed again last night. Having swapped the Teltonika for the RUT240 model I have configured it with the following:
The router is showing connected and ready to go in the event of a WAN1 failure. That being said, I have also configured the WAN2 settings to the following:
Connection Type: Direct
IPV4 Config: Auto
Having got the IP address of the Teltonika, 192.168.1.1, I have input a "Flow Preference" as below:
The good news is I can now see the Teltonika on the network internally using a client PC.
The only issue I now have, I can't access the Teltonika from the VPN. I have input the destination in the client routing page on the VPN, but the page just times out. If anyone can shed some light on this that would be great.
Will test the connection tonight and see if it drops.