iPhone 6s won't log in to Apple ID, won't be recognised by computers and won't connect to Wifi and Bluetooth is 'unavailable'

Hi,

My iPhone 6s will not sign into Apple ID. On the home page of Settings, a prompt appears saying 'Update Apple ID Settings, some account services require you to sign in again' and asks for my password, which I enter (I know it's correct) and when I press 'Sign In' both the 'Cancel' and 'Sign In' buttons turn grey and a processing spiral appears in the password bar, and does not go away at all. So I cannot log in to Apple ID.

A similar thing happens when I press 'Reset' under 'General'. Settings freezes and after a few minutes, crashes. The same thing happens when I try to log in to apps like Facebook and Messenger.

On top of this, it cannot connect to Wifi. When these issues started my phone disconnected from my home wifi and when I try to reconnect by entering in the password (which again I know is correct) a window showing 'Unable to join the network' appears immediately. I have tried to connect to other wifi networks and get the same result, so it is not my home wifi's issue.

Also, the 'Bluetooth' tab on the home page of Settings is greyed out and says 'Unavailable'.

When I try to connect my phone to iTunes on my MacBook Air (macOS 10.14.1) an iTunes window appears saying "iTunes could not connect to this iPhone. The value is missing".

When I try to connect my phone to iTunes on my Windows 10 desktop, the same thing happens, the window saying "iTunes could not connect to this iPhone. An unknown error occurred (0xe8000015)." I also have to 'Trust' the computer every time I unplug it and plug it back in (not sure if that's normal or if it should remember trusted computers?)

Location services and App Store are also unavailable.

As a result of not being able to connect to Wifi or a computer, I cannot update my phone to a more recent OS (currently 12.1).

I've obviously tried restarting it many, many times, which doesn't help. At this point I would Factory Reset it but I want to save the data on it like photos.