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 your finger downwards starting from the top of the screen.
Press the settings icon.
Press Network & Internet.
Press Wi-Fi preferences.
Press the indicator next to "Turn on Wi-Fi automatically" to turn the function on or off.
Press the Home key to return to the home screen.