iPhone connects to the Internet whenever necessary, using a Wi-Fi connection (if available) or your carrier’s cellular network. For information about connecting to a Wi-Fi network, see Connect to Wi-Fi.
When an app needs to use the Internet, iPhone does the following, in order:
Connects over the most recently used available Wi-Fi network
Shows a list of Wi-Fi networks in range, and connects using the one you choose
Connects over the cellular data network, if available
Note: If a Wi-Fi connection to the Internet isn’t available, apps and services may transfer data over your carrier’s cellular network, which may result in additional fees. Contact your carrier for information about your cellular data plan rates. To manage cellular data usage, see Cellular settings.