You can set your mobile phone to use mobile data automatically when the connection to the Wi-Fi network is weak. Before you can turn automatic use of mobile data on or off, you need to turn on mobile data.
Tap Wireless & networks.
Tap the indicator next to "Wi-Fi" to turn on the function.
Tap the menu icon.
Tap 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.