Turning off mobile data: method used to work but now doesn't
It is possible to toggle mobile data without root, using Tasker and AutoInput plugin.

Important: This feature requires screen to be on.
(Some users report that it only works if screen lock security is set to none or swipe.)

You need to perform two tasks as follows:


A1: Go to new action >> Plugin >> AutoInput >> Unlock screen
A2: Net >> Mobile data >> [Set Off or toggle (whatever you wish)]
Exit to save Task


Note: AutoInput also  emulates user swipes on the screen, which means that you can pull down the notification menu and then toggle whatever you want, including mobile data.

Create a condition/profile that suits your needs.

Tested on Android 4

Acknowledgements: 
New to Tasker: toggle Mobile Data + GPS on non-rooted device