If your tablet uses a large amount of mobile data, there may be several causes of the problem.
If mobile data usage for applications is turned on, the applications will use mobile data when your tablet is not connected to a Wi-Fi network.
Solution: Turn mobile data on or off
If automatic mobile data usage is turned on, your tablet will use mobile data when the connection to the Wi-Fi network is poor.
Solution: Turn Wi-Fi Assist on or off
If notifications are turned on, your tablet will use mobile data when applications send push notifications and is not connected to a Wi-Fi-network.
Solution: Use notifications
If GPS position usage for applications is turned on, your tablet will use mobile data when an application uses GPS to determine your position and is not connected to a Wi-Fi-network.
Solution: Turn GPS on or off
If automatic update of apps via mobile network is turned on, your tablet will use mobile data.