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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 your finger 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 "Allow use of mobile data when Wi-Fi is connected" to turn the function on or off.
3. Return to the home screen
Press the Home key to return to the home screen.