Wi-Fi assist one of the new features of the update iOS9. This feature switches an iPhone’s data connection to the mobile carrier when it’s experiencing poor Wi-Fi signal. If you have an unlimited data plan, it’s great – it ends the need to turn off Wi-Fi when the local connection goes south and you can no longer load that hilarious goat video you found. It’s not so great, however, when you have a pretty low data cap and it switches from Wi-Fi to cell without your knowledge. If you’re part of the latter group, head to Settings > Cellular then scroll all the way to the bottom of the page to turn off Wi-Fi Assist to keep you from mobile data charges.