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 Wi-Fi Assist on or off, you need to turn on mobile data.
Step 1 of 4
Find "Mobile Data"
Tap Settings.
Step 2 of 4
Find "Mobile Data"
Tap Mobile Data.
Step 3 of 4
Turn automatic use of mobile data on or off
Tap the indicator next to "Wi-Fi Assist" to turn the function on or off.