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 two fingers downwards starting from the top the screen.
Tap the settings icon.
Tap the indicator next to "Wi-Fi" to turn on the function.
Tap Wi-Fi Assistant.
Tap the indicator next to "Automatically Select the Best Wi-Fi".
Tap the indicator next to "Auto-Switch to Mobile Network" to turn the function on or off.