Hah. This bit us when I got my mother an iPhone SE (2016) to replace her iPhone 4 a year or so ago. I tried to restore from iCloud backup and it kept failing, and finally it dawned on me that the OS may have been out of date. Skipped the restore, updated the OS, and wiped the phone. The restore worked correctly.
On the flip side, the Apple guys have a lot of patience to deal with my stubborn ass trying to activate an iPhone 4... the non-SIM servers were taken offline years ago so I popped in a SIM and off I went.
