Apple’s WiFi Assist feature that was released in iOS 9 has come under a fair bit of criticism for using large amounts of people’s monthly data allowance.
Apple recently released iOS 9.3 beta and there are some changes to the WiFi assist feature, it will now show you how much of your data you are using.
WiFi Assist is designed to switch you from WiFi to cellular data when you have a slow WiFi connection, Apple’s support page for the feature says that ‘you might use more cellular data. For most users, this should only be a small percentage higher than previous usage.’, unfortunately many people have reported that it has actually use a lot of their data.
You can turn the feature off on your iPhone by going to Settings > Cellular in the US and Settings > Mobile Data in the UK, scroll down the bottom and turn off the feature.