That might be the key question to answer for each device. According to the Help Center, the current app requires a minimum of iOS 15. It also says that the actual hardware could be a factor, though. A Macworld article indicates that iPhone 6s and iPhone 7 should support iOS 15 (the maximum supported version for both, according to that source), but according to Wyze that’s still not a guarantee.
I’d still start with determining the iOS version. If you can figure out which version of the Wyze app is on each phone currently, then that would be a good idea, too. Wyze announced over a year ago that upcoming app changes would affect users’ ability to login with older Wyze apps, so I wonder if it’s possible that these people also have really old app versions.
Those are the best answers I can come up with at the moment, and I don’t even use iOS! 