LB2120 4g LTE modem for home Failover not working with other Netgear products

I have been trying for a while, on and off, to try and get an LB2120 4g LTE modem setup for Failover to 4g if my main fixed Internet drops out.

I have a netgear DM200 modem and a R7800 nighthawk router. I am in the UK and am using plusnet as my service provider.

Everything works well without the lb2120, the dm200 acting as just a modem and the R7800 doing all the work as dhcp and router including WiFi.

What I cannot seem to find is a combination that works with the lb2120 I between these.

The lb2120 pickups up the 4g service no problem, ut it has never seen the wired connection. I have tried the dm200 in modem only mode and router mode. The lb2120 has been in bridge mode, router mode, with and without dhcp. Nothing seems to be able to get the wired line to be seen in the lb2120.

Is there a way to get this combo working or shall I just set fire to the lb2120 and get a combined router/lte modem for everything including the failover.

Thanks in advance.