We had a brief power cut last week and it's taken nearly the whole of this week to get my black WiFi extender disks to stop flashing red (no connection).
I think I've finally figured it out. I have one disk in the entrance hall and one in the kitchen. The router is upstairs in the "library." The trick is to start the disks one at a time. When the power was reinstated they, of course, came back on together. Neither could make a connection to the smart hub.
My laptop and mobile phone got a connection (without the disks). Although the signal is weak in the kitchen and hallway the laptop and phone connected to the upstairs router with no difficulty and remained connected (although slow).
Having reset the two extender disks and reconnected them to the router via a network cable and getting a solid blue light moving them back to their respective positions just got me a flashing red response. After a week of recycling the power on both of them to no effect, I was about to throw them against the wall when the one in the kitchen gave me a solid blue light. I realised that I'd unplugged the hallway one in anticipation of the hurling. Plugging that one back in now got a solid blue light.
I'll wait for the next power cut (we get them about monthly) and repeat the experiment to test my theory rather than simulating one now and enjoying another week of BT induced frustration.
If my theory is right, connect them one at a time, then perhaps BT should make more of this in their documentation. On the other hand since this would make life easier for their customers they may decide not to. I've already had to have a moan about their MyBT app which is proving to be a rich source of customer frustration for them to enjoy.
If you get monthly power cuts I take it you will keep complaining to your power company until they resolve the problem which when done will at least stop your monthly frustration lasting a week to connect your discs.
It is up to who you buy the product from to supply what you are paying for. Passing the buck to their supplier does not hold up if you raise a complaint or end up taking them to court.