iPhone 6s set-up on continual loop with iOS 14.3

My dad recently had an issue with his phone, whereas it wouldn't read his sim card. We restarted it, called the company, even tried other sims, to no avail.

Frustrated as he has a lot of his work on his phone, he asked for another family member's phone (said family member doesn't use it much) to have a working phone while we waited for a new one to come.

This borrowed iPhone is a 6s, and everything is fine until it forces you to update to iOS 14.3, it will download and say it is verifying the update. After that, it just stops and asks for you to press "Download and install" again. We've spent all day trying to do this.

According to the bottom of this page, iOS 14 should be compatible with a 6s, however many of the discussion forums state otherwise. Is there a way to bypass this seemingly forceful update? Why does it not update?

I'm kinda at a loss at what to do, as not only my dad has no phone but now we've left this family member kinda phone-less too.