Starting with iOS 9, Apple introduced a feature in iOS called Wi-Fi Assist. When enabled, Wi-Fi Assist utilizes your cellular connection whenever your Wi-Fi service is weak, ensuring a seamless Internet connection. For instance, if you were visiting a webpage on your iPhone and your Wi-Fi connection became weak, Wi-Fi Assist would step in and use your cellular connection to ensure the webpage would load without issue.
While a good feature, not too long after the introduction of Wi-Fi Assist, there were reports of users experiencing unusual data consumption with their cellular data plans. The recommendation to users running iOS 9 was to turn off Wi-Fi Assist to avoid unintentionally burning through their cellular data. Though this recommendation came out several years ago, it still holds true today with iOS 11.
By default, Wi-Fi Assist is enabled in iOS 9 and later. If you want to check to see if your Wi-Fi Assist setting is enabled or disabled and/or if you want to disable Wi-Fi Assist to avoid unnecessary data consumption, go to Settings then Cellular. Under Cellular, scroll down to the bottom of the page until you see Wi-Fi Assist. There will be a toggle switch next to Wi-Fi Assist. If the toggle switch is green, it means Wi-Fi Assist is enabled. If the toggle switch is white/off-white, it means Wi-Fi Assist is disabled.
If you’ve been experiencing unusual data consumption, check your Wi-Fi Assist settings. It may be the culprit!