Cause 1 of 1: Notifications are turned on
If notifications are turned on, your phone will use mobile data when applications send push notifications and is not connected to a Wi-Fi-network.
Solution: Turn off notifications.
1 Find "Notifications"
Slide your finger downwards starting from the top of the screen.
Press the settings icon.
Press Notifications.
2 Turn notifications on or off
Press the required app.
Press the indicator next to "Block all" to turn the function on or off.
3 Return to the home screen
Press the Home key to return to the home screen.