Sony Xperia XZ3
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.
Slide two fingers downwards starting from the top of the screen.
Press the settings icon.
Press Network & Internet.
Press Wi-Fi.
Press Wi-Fi preferences.
Press Advanced.
Press the indicator next to "Auto network switch" to turn the function on or off.
If you turn on the function, press ENABLE.
Press the Home key to return to the home screen.