“I can’t even turn the phone off when that happens. I have to let it keep rebooting, on its own, until the battery runs out of power.”
I had a problem like that years ago. Removed battery, replaced and let it charge overnight.
It’s worth a try. It’s an Android but from a couple years ago.
There’s another that worked fine but I wanted this (now corrupted) phone. So I idled that phone for almost a year.
It (the idled phone) was fine ........ until I resurrected it. VERY BAD on voice, text and internet connection. A complete waste of time to even try to call someone.
Finally found a comment (elsewhere) where they suggested:
1) Power the phone off
2) Remove the SIM card
3) Wait at least 30 seconds
4) Reinstall SIM card.
5) Reboot the phone.
By golly it worked on that phone.
I may try similar with this phone after your suggestion. I’m fairly-certain it has a removable battery. I’ll have to charge it up first.