Access Point Name, or APN, is a set of valued data that assists your phone in connecting to a network provider's internet. AT&T is a provider in the United States for both 3G and 4G networks. The values to create an APN are name, proxy, username, password and server.
The network path used by iPhone devices for cellular data connectivity is the Access Point Name defined. On an iPhone, you are able to view and edit the APN settings depending on the network provider. An iPhone connected to AT&T network is able to create the Access Point Name through which the device accesses the cellular data connection.
To create an APN, it requires users to get the values from the Internet and type the required fields. The APN settings are viewable on iPhone in Settings > Cellular > Cellular Data Network or Main Screen > Settings > General > Network > Mobile Network; this depends on the iPhone model. Note that some mobile carriers do not allow the default APN settings to change. Selecting the correct APN settings is crucial because any incorrect information, the iPhone loses functionality in cellular data connectivity. Apn-Settings.com gives correct information in case you want to edit the APN settings, both for 3G and 4G.
When creating the AT&T Access Point Name, leave the username and password section blank while filling the information from the Internet, save the settings and set as default. The iPhone is able to access the cellular data from AT&T carrier. Tutorials videos on setting APNs are found on YouTube.