Hidden deep in iOS 9 and later is a setting that Apple intended to improve your iPhone experience. Called Wi-Fi Assist, it enables iOS to use cellular data automatically when Wi-Fi connectivity is poor. That’s a nice idea, but it also means that your iPhone (or cellular iPad) may use lots of expensive cellular data when you’re not expecting it to. If you worry about cellular overage charges, turn this feature off in Settings > Cellular > Wi-Fi Assist (it’s near the bottom).