Getting a USG with a Spark 4G modem

I've a remote location that I have a PC and other equipment and have a 4G modem to connect to. I need to be able to remotely see how the equipment is working and VPN directly when required.

I have an ERL and Unifi at home so I went with Unifi for compatability and reliablity.I have an Unifi USG, Managed 8 port switch with POE, Cloud key and AP.I can get this all working with the spark supplied B315 modem except with the USGIt refuses preventing internet access with the message "Gateway is unable to connect to the Internet"It does show some of the IP address detail but no option to proceed.

If I bypass The USG it and connect the modem to the switch, the USG can be seen by the controller but when I try to adopt it, it mandates a firmware update which never completes.

I've tried changingthe LAN to 192.168.10.1 on the controller to avoid a conflict but no change

I thought it may be a DHCP conflict so turned that off on the modem but then everything looses internet.

I know the B315 doesn't support bridging so I asked spark and they suggested a static IP address which I now have but still no change.

When I go into the modem now all the network options are not populated so can't change DHCP.

Any ideas how I can get this to work?

Please help - I'm ready to defenstrate the USG