LTE5388 WAN failover and IP passthrough

Mack1979

My network:

BT Openreach modem (ADSL2+)

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LTE5388

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Google Wifi Router (ethernet and mesh connection to rest of network)

Google Wifi doesn't support WAN failover natively, nor double NAT. I want to set up the LTE5388 for WAN failover as well as passthrough so the Google router can handle NAT, DHCP, PPoE and routing.

The LTE supports WAN failover, but only if the LTE also acts as the router by default. It also supports cellular passthrough which works, but doesn't support WAN simultaneously (it even disables the options to enable WAN on the LAN1 port).

I know others have set this and other Zyxel routers up to work the way I want with Google Wifi, but I'm struggling to find the right combination of settings. I suspect I need to use maybe WAN failover but then disable DHCP, NAT etc on the LTE, but I haven't found the right combination which works (and in doing so I also lose access to the LTE settings once I disable DHCP as I can't connect to it directly then, nor does it show with an IP address on my network.

I asked support and they kind of shrugged their shoulders and said they'd look into it.