So open smart actions, hit the green + button to add a new action. Then choose start from scratch. In the trigger category hit the + and add "wifi connection", its towards the buttom, for me it showed a list of all networks that I have connected to. Select the networks you want this action to apply to. If you want it to apply to all wifi connections select all the networks listed. After adding that you will want to go into the actions by hitting the +. Scroll down to button you want to select cellular data. it will pop open a menu with on or off option. Select off. What this makes is. When connected to whatever networks you selected it will turn of cellular data. If you add new networks later on you might have to make these changes in smart actions.
And remember you can't send or receive mms (i.e. picture messaging) when connected to wifi.
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