If you are an iPhone user who has experienced major spikes in data usage since your upgrade to iOS 9, fear not. This tutorial will walk you through how to turn off the new Wi-Fi Assist feature of iOS 9, which automatically switches your phone from a WiFi network to data when WiFi signal is poor.
How To Disable Wi-Fi Assist in iOS 9
Go to Settings > Cellular. Scroll to the bottom of the page until you reach the Wi-Fi Assist option.
Simply toggle the Wi-Fi Assist switch Off to disable this feature.