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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 Wi-Fi Assist on or off, you need to turn on mobile data.
1. Find "Mobile Data"
Press Mobile Data.
2. Turn automatic use of mobile data on or off
Press the indicator next to "Wi-Fi Assist" to turn the function on or off.
3. Return to the home screen
Slide your finger upwards starting from the bottom of the screen to return to the home screen.