It’s wrecked both my OH’s iPhone and iPad Mini.
Rang O2 yesterday because she’d to keep enabling / disabling 4G to get a reliable data signal and because the MMS option was greyed out. They had her reset settings, lo and behold it started working.
Soon as she got off the phone it started locking up hard / spontaneously rebooting. Ended up doing a factory reset on it (when she finally got to the menu option before it rebooted again). Following that it was better but still unstable, a second round of ‘reset settings’ and – touch wood – it seems to have sorted it out.
It’s now better than it’s ever been. So, anecdotally from a sample size of “one” I’d suggest trying a factory reset followed by a reset of settings. Make sure you’ve got an iCloud backup first, of course.
Might have to try the pad next.