Apple’s new iOS 9 has a lot of improvements and some really useful features, but unfortunately, one of them may be killing your data plan without you knowing it.
iOS 9’s new WiFi Assist feature will automatically switch to your mobile network for data if you’re experiencing a crappy WiFi connection. Unfortunately, since there’s no prompt for this switch, there’s a chance that your data plan is being depleted even while you’re sitting at home or at the office in a WiFi zone.
Since this WiFi Assist is on by default, you should really consider turning this off completely. To do so, simply head to Settings > Cellular and check off WiFi Assist all the way at the bottom.