One of the nicer features in iOS 9 is the ability to automatically switch to using cellular data if your Wi-Fi is a little sketchy. The not-so-nice thing is the fact that this feature it is switched to ‘On’ automatically when you upgrade to iOS 9.
There has been some grumbling from users (including me) about the potential, and sometimes real, adverse impact to data usage. I can’t say with absolute certainty that this was the culprit but our data usage last month was a good bit higher than usual.
If you decide, like I did, to turn this feature off; in your Settings app, tap Cellular, scroll all the way down to the bottom to ‘Wi-Fi Assist’ and turn it off by moving the slider to the left.