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 Wireless & networks.
Press the indicator next to "Wi-Fi" to turn on the function.
Press the menu icon.
Press the indicator next to "Wi-Fi+" to turn the function on or off.
Slide your finger upwards starting from the bottom of the screen to return to the home screen.