Have you used smart actions? Just open it up and play around with it, its relatively easy. I think there's even a preset for it
Hmm. I created one triggering no phone movement, and the actions are set mobile data off. set wi-fi on. is that backwards to what i'm trying to do? for some reason the Smart actions are a little confusing to me. when i pick up the phone it goes to wi-fi only when I am in my home.
I would set the trigger as screen off and have the action as disable mobile data. Make sure in the phone settings under display that you have the phone set to go to sleep after 1 minute of inactivity