Hello Tom,
I can help clarify setting up a WAN failover with the new 7.x firmware UI.
Here at our Wiki (RUTX11 Failover - Teltonika Networks Wiki) you can find an example on how to setup specific WAN failover for interface priority & IP to check functionality against.
Below can be done while using 07.07.1 Firmware.
If you are logged into the WebUI and have the configuration option toggled to Advanced, you should also be able to start a simple failover from the Network → WAN section, if any of the interfaces are disconnected.
Here with the default out of the box configuration, the interfaces will try to reach out to their default route for any type of reply.
You can toggle which interfaces you would like to have failover, say for example primary WAN interface then 2ndary Mobile interface. And you can drag the interfaces up or down in priority, with the interface at the top with the highest priority.
Make sure to hit Save & Apply for either configuration option you choose at the end for changes to take effect.
Please let me know if you need any additional clarification.