Unable to update Apple ID or download latest software

Mother in law has asked me to look at her iPhone SE (MX9U2B/A) and iPad mini 4 (MK9Q2B/A). I'm unable to get either device to sync with Apples servers to either update the software or accept a new Apple ID password? The iPad mini and iPhone are both stuck on V.15.3.1 and won't update to V. 15.4.

I had to set a new password (this has been an ongoing issue for several months) we followed the instructions, confirmed her mobile number and 28 digit recovery key and then set a new password. We are now unable to get either device connect and download the latest software and instead keep getting the error message 'Verification Failed An unknown error occurred'. This now also appears to affect iCloud also - but I guess that makes sense.

I've tried resetting the network settings (on the iPhone) and hard reset both devices however this hasn't made a difference. Also both devices has sufficient available storage iPad = 104Gb and iPhone = 42Gb.

I'm tempted to delete both devices and start again but worried about what's saved and what's not.

Now fed up with this issue - any help much appreciated!