iOS 9 includes a new feature designed to optimize network performance by automatically switching to a mobile data network if a poor Wi-Fi signal is detected or Wi-Fi responds painfully too slow. It’s called Wi-Fi Assist and comes enabled by default on iPhones and cellular iPads running iOS 9 or iOS 9.0.1.
As first noted by Quartz, some iPhone owners with poor Wi-Fi reception are seeing dramatic increases in their mobile data usage after updating to iOS 9.
For example, some folks are reporting a jump from one to as much as seven gigabytes since updating to iOS 9. To avoid unwanted overage fees on your wireless bill, disable Wi-Fi Assist with just a few taps, here’s how.
People who don’t have unlimited cellular data are especially advised to turn off this new iOS 9 setting. At any rate, anyone who has upgraded their iPhone from an earlier iOS version to iOS 9 should know that iOS 9 automatically turns on Wi-Fi Assist.
To check out your current cellular data tally, head to Settings > Cellular.
How to disable Wi-Fi Assist in iOS 9
Step 1: Open the Settings app on your iPhone or Wi-Fi + Cellular iPad.
Step 2: Navigate to the Cellular section. If you’re outside the United States, the Cellular label might read Mobile Data instead.
Step 3: Scroll down to the bottom and flip the Wi-Fi Assist switch to the OFF position.
Full tutorial: How to set up and use Wi-Fi Assist on iOS 9
When Wi-Fi Assist is enabled, your cellular-capable iOS devices will “automatically use cellular data when Wi-Fi connectivity is poor,” says Apple.
To further minimize mobile data usage, disable data hog apps and services in Settings > Cellular by turning cellular data on or off for any app that can use cellular data.
If a setting is off, iPhone uses only Wi-Fi for that service. In addition, Settings → App and iTunes Stores let you disable mobile data for both iTunes Match and automatic downloads from the iTunes Store and the App Store.
You can tell that Wi-Fi Assist has kicked into action when the Wi-Fi signal icon turns gray before the cellular data dots appear in the iOS status bar.