The new iOS 9 is eating up your data plan. iOS 9 use your phone’s data signal to strength weak wifi signals. However, it can burn up your data plan real fast if you’re regularly in weak wifi spots.
If your wifi signal is low, it can uses mobile data to increase the signal strength.
Here you will come to know to how to fix it.
Go to setting on your iPhone.
Scroll down all the way to the bottom and disable “Wifi Assist”
Now wifi will not use cell data.
You can always turn it back on but it is recommended to leaving it off by default.