All three buttons on iPhone XS malfunctioning

Very weird issue has developed on my XS. Upon start-up the 'up' and 'down' volume graphics briefly show onscreen, as if the buttons are being pressed, when they're not, then within 60 seconds the phone goes into a hard reset via the apple logo, as if the on/off side button had ALSO been held down (when it wasn't). It's as if all three buttons are malfunctioning simultaneously (which I realise is highly unlikely, so it'll be a fault elsewhere). Here's where the fun starts... because the phone can only be on for 60 seconds each cycle, there's insufficient time to update the software (it's currently on 13.5.1), or frankly to do anything else (including, I guess, a total reset). I looked for an option to disable the buttons, or the hard reset function, and unsurprisingly they don't exist. In the good old day's I'd have dropped into an Apple Store to get it looked at, but here's the hilarious thing, I've got to self-isolate for another 9 days having returned from France and I'm pretty stuck without a functioning phone.... I get the impression I may just be out of luck, but any ideas/suggestions would be gratefully received!