The iOS9 update for iPhones may be a data nightmare for some... eating up their data quota for some in a day...
When Wi-Fi Assist is turned on, it will supplement a weak Wi-Fi signal by switching to cellular data. For example, let’s say you are trying to load bus schedules while running out of your house; iPhones on the old iOS would cling on to your home’s Wi-Fi, until the signal was too far away to stay connected. However, this would result in very slow load times due to weak signal. In iOS 9, Wi-Fi Assist will sense this weak signal and switch your phone to cellular data—effectively solving the issue of spotty Wi-Fi.
This sounds useful, but it’s a potential problem for people anyone without an unlimited data plan (which is a lot of people). If your house has rooms that are Wi-Fi dead zones, Wi-Fi Assist will cause your phone to use cellular data even though you have access to Wi-Fi. So if you don’t have cellular data to spare, make sure you turn Wi-Fi Assist off. To do so, go to Settings > Cellular and scroll all the way to the bottom.
There you’ll see Wi-Fi Assist switched on by default. Turn off Wi-Fi Assist, and enjoy the peace of mind that comes with not wasting data. RIP, Wi-Fi Assist.