RBR750 Satellite flapping between router and satellite for backhaul

I've no idea of the sq.ft. area of the house, but there are brick walls all over the place and it's L-shaped. The signal between satellites goes through two external double-brick walls and two internal walls (one lath and plaster, the other plasterboard), and the same plus a brick stairwell and a ceiling/ floor when it decides the router is a better bet. The straight-line distance is around 30ft and 45 ft respectively. Some home automation, an Echo 5, the boiler control and a SONOS speaker had flaky connections out that way, which is why this satellite has been installed-modifying the thermostat temperatures used to fail all the time. It doesn't now. Calls also dropped as you enter the room below. Very annoying!

I did not reconfigure from a factory reset after the latest updates as frankly, that as a requirement is insane. I use a /30 subnet to uplink to my router, and quite a lot of reservations-for home automation/ IoT hubs and various other bits of config. Why would I consider resetting to factory, unless the software is shoddy?

Sky Fibre broadband router via ONT, using their router with all IPv6 configuration turned off, all Wi-Fi and any other service turned off with the exception of ALG/SIP, DNS, NTP, routing and a LAN port configured to use the other address in the /30.

2.4Ghz: Auto
5Ghz: 36

I can see only 3 other SSIDs when I scan.

I assume that I'd only lower the 5Ghz radio, we connect from 90ft down the garden to the satellite, which probably uses 2.4Ghz). I need to keep that.

I'll try the radio output reduction now and see if that helps. I could also look to move the satellite that is flapping further away and down another floor and see if the backhaul still connects at 5Ghz. That would be about 60ft away then.

Thank you.