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Wi-Fi Assist automatically uses cellular data when your Wi-Fi connection is poor. This is a helping hand that stops patchy Wi-Fi from being so annoying. No more will your iPhone fruitlessly try to load a page on a terrible Wi-Fi connection when you have the option of 4G.
This feature is enabled by default, and it works automatically.
Turning off Wi-Fi Assist
However, if your data plan limits your cellular usage (most do) you should be careful as this feature can use up your mobile data allowance quicker than usual. In most cases it shouldn’t be a big deal, as Wi-Fi Assist isn’t used for large or background downloads. It only kicks in when you’re actively using an app requiring a solid Internet connection.
However, if you would prefer not to run the risk, you can disable the feature entirely. Open Settings and tap Cellular (or Mobile Data in some regions). Scroll all the way to the bottom of the page and toggle Wi-Fi Assist to the off position.
Underneath this option will be an indication of how much cellular data Wi-Fi Assist has used to date. This may help you decide whether or not to continue using the feature.