In the iOS 9 update, Apple has made it so by default, your phone is allowed to use cellular data while connected to wifi. What this means is when you're using your wifi at home, it can increase your speed by using your culleler data. This pretty much burns your data for no reason.
We'll cover how to fix this in this two step, super easy, instructable.
That's it, now your Phone won't waste your data while at home.
If you have any problems, please leave it in the comments and I will try to help.