I have had a running issue with my new fibre installation.
It was connected in March and since then I have had random connection issues.
I have had a number of engneers out who have done various things but to no avail.
I finaly identified that the issue seemed to be that the 2.4 SSID would randomly vanish. Devices that could would switch to the 5g SSID. Those that could not would of course not be able to connect to the internet. The wired devices would also be ok.
If left alone the 2.4 SSID would return so by the time I got round to fixing my wife's issue it would be OK again!
As WIFI is different for every location, it's not possible for BT to guarantee how well it will perform. This is the case with all internet providers and not just BT
It is correct that BT and all other ISP do not guarantee wireless connection. There are far to many variables involved that are out side the ISP control.
You could try selecting channel 13 on your 2.4Ghz frequency. This is usually the least congested channel. If you get a warning about using it just ignore it. The warning only applies in USA.
See link how to do that.