Adding a secondary / failover gateway on a different subnet - how?

[iankearns]

Hi all

Am using OPNSense in my home, likely overkill but I like the way it works and it has a strong support base.

Currently have two service providers supplying internet connectivity; primary is cable provider at 200Mbps which we use for most activities and has the home network attached, the smart devices and also the two AP's around the house. This is the one that the OPNSense device is located. Modem<>OPNSense<>Switch<>Netwrok. This is all on a 192.168.250.0/24 network

Aside from that we have a FTTC connection provided by another ISP. Lower bandwidth speeds 40/10Mbps and we just simply use the ISP supplied router and WiFi on the router. This is mainly used for my partners work. This is on 192.168.2.0/24

I would like to be able to use the FTTC service as a failover should the cable / primary be unavailable however when I have tried to cable the FTTC router back to OPNSense and a free network port on the PC running OPNSense and setup as a gateway then it doesnt seem to work. I have amended the DHCP scope being served on OPNSense to include a wider class B subnet but still then it still doesnt work.

Can anyone offer any pointers on this? I have looked into virtual IP's and wondered if this was a solution but got myself in a bit of a muddle. Would it be easier if I re-IP'd the FTTC router and just give it a really narrow DHCP scope of a couple of IP's as my partner really needs to use that connection for her work and I dont want the FTTC router to become a default gateway for all normal network traffic, only when the cable ISP is down.

Thanks all