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 notifications on or off
1. Find "Apps & notifications"
Slide two fingers downwards starting from the top of the screen.
Slide two fingers downwards starting from the top of the screen.
Press the settings icon.
Press Apps & notifications.
Press See all ‹› apps.
Press the required app.
Press Notifications.
Press the indicator next to "Show notifications" to turn the function on or off.
Press the Home key to return to the home screen.