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You can set your phone to use mobile data automatically when the connection to the Wi-Fi network is weak. To turn automatic use of mobile data on or off, you need to turn on mobile data.
1. Find "Wi-Fi preferences"
Slide two fingers downwards starting from the top of the screen.
Press the settings icon.
Press Network & Internet.
Press Wi-Fi preferences.
2. Turn automatic use of mobile data on or off
Press the indicator next to "Switch to mobile data automatically" to turn the function on or off.
3. Return to the home screen
Press the Home key to return to the home screen.