Wi-Fi Assist is a new feature in the new iOS 9. This enabled feature will automatically use Cellular data when Wi-Fi connection is insufficient.
iOS 9 can use mobile data to increase your WiFi signal strength. However, people have noticed this feature use up their Data plans… This is really a big enough problem that Apple just go sued $ 5 million. Here is how to turn it off.
Setting > Cellular > Scroll download to the bottom > Disable Wi-Fi Assist.
Now Wi-Fi won’t use cell data. You can always turn it back on, but we recommend leaving it off by default.