Failover to 4G

[kermitxyz]

Aim - Fibre failover to 4G if fibre fails

I can see two ways:

1. Have both as a gateway, but with different priorities
2. Use the Gateways > groups option

Which is better practice? 

Secondly...

On OpenWRT the 4G router was connected to the multiple port network card in the router and on a different subnet.   

With OPNSense, should I do the same (although I am not sure *how* yet) or should I just give the 4G router an IP address on the same subnet, connect it to my switch itself and create an interface with that IP address?

thank you for any advice :)

[kermitxyz]

Anyone ??  :)