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.
Slide your finger downwards starting from the top of the screen.
Tap the settings icon.
Tap the indicator to turn on the function.
Tap the menu icon.
Tap the indicator next to "Switch to mobile data" to turn the function on or off.