I think you need to be using at least Wyze app v2.45 in order to login. On my Android device I was able to login with Wyze app v2.40, but I don’t know if this is the same for Apple devices.
Your iPhone 6S on iOS 15.8.3 should be able to login to Wyze assuming your Wyze app v2.45 or newer.
Your iPhone 6 Plus won’t be able to login to Wyze because the iOS 12.5.7 is too old and Apple’s App Store won’t let you install Wyze app v2.45. My iPad Air (1st gen) is also on the same iOS 12.5.7 and it only let me install Wyze app v2.31.1 so I can’t login as well.