What's important is that this failover device needs to support the existing PPPoE that's in place as my router (Netgear R9000) is responsible for doing that.
Are you saying you want to keep the Netgear R9000 handling the PPPoE session over the FTTC link to the primary ISP? If so, can you explain *why* you need this?
The device which is best placed to make a failover decision ("the PPPoE link is down, send traffic down the alternate path") is the one which terminates the PPPoE session, as it can see the PPPoE keepalives.
Hence the best solution would be to have a single router with two WAN ports: one PPPoE to the FTTC provider and one to the 4G/5G network. If the R9000 supports dual WAN with failover, in principle it would be possible to put a 4G/5G router upstream of the second WAN port, with a lower priority default gateway pointing to the 4G/5G router. I have no idea if the R9000 is sophisticated enough to allow this, at least with its standard firmware (OpenWRT could probably do it though).
When you say "failover to the cellular connection", do you mean you already have a cellular connection available, and if so what is it? (What provider, what device) Or is this something you have yet to choose?
The LM1200 needs a pure IP connection i.e. without any form of PPPoE encapsulation to pick up the FTTC connection.
Which is exactly what your R9000 provides: a pure IP subnet behind the router.
However, any device behind the R9000 would not be able to tell reliably whether the FTTC link is up or down. You'd have to configure it to do ping tests to 8.8.8.8, say, via the ethernet link to the R9000, and to change its routing dependent on whether those pings succeed or not.
If it were me, I'd be looking at a Mikrotik router to terminate the PPPoE, with either a USB modem or 4G ethernet router for the backup link - or even better, one of the Mikrotik models with built-in LTE. But that means getting rid of the R9000, or delegating it to being a wireless access point.