Huawei P20
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Turning automatic use of mobile data on or off
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.
Press Settings.
Press Wireless & networks.
Press Wi-Fi.
Press Wi-Fi+.
Press the indicator next to "Wi-Fi+" to turn the function on or off.
Press the Home key to return to the home screen.