You can cut data roaming costs and save on your phone bills by changing cellular data settings. To use the optimal connection method, change the Wi-Fi and cellular networks settings. Data roaming means using your phone to receive data over networks that your network service provider doesn't own or operate. Connecting to the internet when roaming, especially when abroad, can raise data costs substantially. Using a Wi-Fi connection is generally faster and less expensive than using a cellular data connection. If both Wi-Fi and cellular data connections are available, your phone uses the Wi-Fi connection.
Swipe down from the top of the screen, tap network_cell and switch
To follow your data usage, tap