When Apple released the new iOS 9 operating system, it came with a bunch of new great features. However, there’s one feature that will hack away at your data if you don’t change its default setting.
Apple’s new “Wi-Fi Assist” feature will switch your phone over to cellular data if it senses the Wi-Fi network you’re on isn’t strong enough. Therefore, you can be using your cellular data without even realizing it and your phone bill is not going to be pretty.
Fortunately, you can easily turn off this setting by going to Settings > Cellular. Then scroll to the bottom and turn the “Wi-Fi Assist” toggle off. Here’s an image for reference.
That should help with your data plan!