When you need an internet connection, iPad does the following, in order, until the connection is made:
Tries to connect to the most recently used available Wi-Fi network
Shows a list of Wi-Fi networks in range and connects to the one you choose
Connects to your carrier’s cellular data network (Wi-Fi + Cellular models)
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 View or change cellular settings on iPad.