Failover with open wifi networks (e.g. BT Wi-Fi)

Some context - anyone using a BT router has a BT open wifi network, which anyone can use if you're within range. All you need to do is connect to it, and then log into your account.

I am near such an open wifi network, and would like to set this up as a failover, if for whatever reason my physical wan cable is disconnected from my router.

I have mwan3 set up fine, I just can't seem to get the connection/interface to the btwifi set up correctly. Annoyingly I can no longer find it, but I found a webpage that said I just connect to the network with a laptop with a spoofed MAC address and sign in to BT, and then disconnect laptop, go to openwrt and connect with wifi and just put the BSSID as the spoofed address. I then get a bit stuck from here on in.... do I set up the interface as a static IP for BT?

I have looked online, but all the videos point towards using only the BT wifi as a singular WAN connection, rather than a failover. Any help would be much appreciated. Thanks!

Many of these systems use a captive portal and require a web browser to enter credentials. If that is the case, this will likely not be possible to do via openwrt, especially with an automatic failover method.

Yeah it does require a captive portal. Ok, no worries on that then!

What about if I wanted to set it up as a sort of manual failover? I want to be able to have the dual wan set up, and use the bt wifi when needed. Could I set up the btwifi wan, and then it have it's own wifi network I sign into to enter credentials, and then the router has access to the accepted portal/wan?

isn't your main ISP also BT ?! - i suspect that in 90% of the maintenance cases/issues both you and your neighbor btwifi will be affected so I see very little practical use

Maybe I just want to do it for fun and learning something new!