Prevent mobile data being turned off
Go to Settings > General > Restrictions

If you have setup Restrictions, enter your Restrictions passcode. 

If you have not set it up yet, enter a password (twice) and enable Restrictions.


Make sure you will never forget this passcode; it is required even when resetting the phone. If you forget it, not only will you be disallowed from entering the Restrictions section, you will also not be able to reset your iPhone.

On Restrictions page, go down to the “Allow Changes” section and click on “Mobile Data”.

Set “Mobile Data” to “Don’t Allow Changes”. 


Leave the “Data Plan” to “Allow Changes”.

Open control center once and close it.


For iOS 12 here are the instructions