Help needed setting up openwrt

Hi all,
Firstly sorry for any poor spelling / grammar, my English isn't the best.

I was wondering if someone could help me with a Idea I have to help my elderly neibour.

The problem is that they have very unreliable internet and because of covid they stay at home and only do online shopping.
I own the flat next door but don't live there but I have a very good internet connection at that flat, what I'm trying to do is to use a raspberry pi4 as failover router so when their internet drops they can use my guest WiFi automatically.

I currently have it working using mwan3, usb ethernet and WiFi dongle, on my test setup everything works fine and it switches over to my test hotspot, but the issue is the my flat is about 100miles from where I live so I need to get it right before I go there.
Currently my neighbor has a virgin media superhub in modem only mode which then goes to a router, what want to do is to put the raspberry pi inbetween the modem and the router but I'm not to sure if this would cause issues for traffic going to CCTV cameras and alarm system.
Hopefully it makes sense.
If you have any thoughts or ideas I would be very grateful.

Just been down to the flat and it works perfectly, I just had to do some port forwarding for the CCTV and alarm.

Thanks for looking.

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