Home wifi wastage for nothing
This article should help you greatly: http://www.wired.com/2015/10/a-new-ios-9-feature-is-eating-your-cellular-data/

There is also a list in settings that lists how much data various apps use.  It's most likely Mail or Messages using so much data for push notifications.  If you switch to a 5-minutely update rather than push, then you might have better results.