Using two external internet routers (4G and BT) in a failover scenario

I've read multiple posts here and elsewhere and believe mwan3 might be the right tool to use in this scenario. I have an OpenWRT router and was hoping to use two external routers as the internet links into this. One of them is TP Link 4G router, the other is a BT provided router.

I have been using my BT line through the openwrt router for a while, and using static routes to use the 4G connection when BT goes down... the problem is the BT line is down an awful lot right now and they are insisting I use their router to fix it. So I was hoping that I could use the 4g and BT routers as internet only (dhcp is disabled) and connect into my openwrt router, with the BT line being the main one and the 4G the backup/failover.

Apologies if this isn't clear, I can try and explain more if needed

I confirm that mwan3 will do what you want.

You will have to create the following interfaces on your openwrt router

WAN (already present by default, add metric 10, from line BT)
Wanb (to be created by removing a LAN gate from the bridge Br-Land, add metric 20 and to be assigned to the Firewall Wan area, from router 4G)

Install and configure mwan3:

What protocol should I use in the instance for WAN? before I moved to using the BT router I had my rj11 connected into the openwrt router - so the WAN is set as PPPoE and would have used the DSL port rather than the "WAN" port on my router.
Apologies for the questions, I am a novice with OpenWRT and learning as I go, I have read multiple guides so trying to expand on my existing setup

It depends on which ISP you use

and if you want to avoid double NAT

see this document:

Thanks, I hadn't considered double NAT. I was over-simplifying this in my mind...
My ISP is BT, they do provide a new smart hub 2 that can be used in "bridge" mode so only act as a modem, I may look at getting one of these, as I am out of contract soon