Not only did I reset to factory defaults, but in shear frustration got a new one out of the box. Did the firmware and then went through the process to add the Wireless Network, and then configure failover to 4G, and got exactly the same results.
The name I used in "Custom" did not work: scstrain, also tried variations of that, and still did not appear in the Failover list, only the standard interfaces.
In desperation I did it all over again for the millionth time, only this time used the custom name: caledonian
And to my surprise this appears in the "Failover" list so I can use it.
Is there a bug in the OS? odd that it wont allow "scstrain" but it will allow "caledonian"
Still no Failover, despite not having it setup correctly (I believe), If I disable the WiFi at source you would hope it will switch over to Mobile, however it doesnt.
Currently going through Factory reset again, when it comes up I will follow the Wizard and only do essential changes before trying again with Failover