New (refurbished) phone can't run System Update or make Samsung Pass work.

I'm having a bit of trouble with my refurbished Samsung S23 Ultra (bought from my phone network provider, so not some questionable secondhand phone place!), some things just aren't working properly. I should probably add I moved from another Samsung so used Smart Switch to move everything over.

Initially I noticed that I couldn't get Samsung Pass set up properly, it was getting stuck on the loading screen after I clicked "Continue". At the time I thought it might be a temporary problem, and everything else seemed to be working OK so I left it for the time being.

I later realised I couldn't set up google Wallet for contactless payments, I tried to authorise the cards I already have stored in Google Pay but got an error message saying my phone didn't meet the necessary security requirements. I eventually figured out that my phone was apparently not "Play Protect certified", which was causing the issue in Wallet. When I looked up how to solve this problem it suggested I may need to update my system, but I then discovered I can't perform an update! When I attempt to run the Software Update I get a message saying "registration failed". 

I've looked up how to solve all these problems but the things I've tried so far haven't helped. I've restarted the phone, cleared the cache for all the relevant things (then restarted the phone again), started the phone in safe mode to test Samsung Pass (it does exactly the same thing in safe mode), and restarted again a couple more times.

Now after doing all that, I've created a new problem, as all of my widgets have disappeared! I can presumably add them all back manually but I'd rather not, as I had quite a few widgets, all customised in various ways so it'll be a tedious process!

Does anyone have any idea what might be wrong with my phone and how to solve any of it?

The only thing I haven't tried which I've seen occasionally as a suggestion is connecting the phone to a computer and trying to update using Smart Switch, as I'm currently working from a Chromebook so I need to go find the family Windows laptop to do that, but that's the next thing I plan to do.

Obviously if nothing works I'll get in touch with the phone company to see if they can help, but I'm hoping I can solve it without having to deal with customer support or send my phone off to be fixed!