Turn off WiFi Assist

If you’ve recently upgraded to iOS 9 on your iPhone or purchased a new iPhone 6S or 6S Plus, you may want to check the new WiFi Assist setting, which is on by default. WiFi Assist allows your iPhone to switch to your cellular connection when your Wi-Fi connection is poor. While this can be a good thing, some users are experiencing large spikes in their cellular data usage causing them to go over their monthly data allowance. To turn off WiFi Assist, go to Settings, Cellular and scroll to the bottom to find WiFi Assist. Touch the toggle switch to turn off WiFi Assist.