How to setup secondary Internet device

I am dealing with a simple scenario of setting up a phone(or 4G modem) in my network that will be serving as secondary Internet connection gateway ready to be switched to when the main connection to Internet fails on the WAN port.

I've been searching for some solution but did not find anything other than mwan3, but still unsure whether that can be used for my scenario.

I have currently RPi with OpenWRT running with 2 NICs (WAN/LAN). I would like the 4G modem/phone to be part of the LAN network. Currently my LAN GW is
Is it possible the have phone on IP eg. and then set it as a main GW in the case the WAN port lost connection to the Internet?
I am already monitoring my network with Home Assistant and I can setup automation for some kind of fail over, but I am not sure how to go on about it in terms of rerouting the traffic. Obviously the LAN bridge should reroute the traffic from WAN bridge to and then switch back when WAN is back on the Internet.
To do it directly in OpenWRT system could be easier, but I want to be also notified about the fail over and I am not sure how easy can be all of that done in OpenWRT and also I don't want to run too many things on it to preserve HW resources as its only RPi B V1 and it sometimes struggles with optimal performance so perhaps external solution to failover might be better solution. Not sure though.