If iPhone is connected to the Internet via the cellular data network, the LTE, 4G, 3G, E, or GPRS icon appears in the status bar. Note: LTE, 4G, and 3G service on GSM cellular networks support simultaneous voice and data communications. For all other cellular connections, you can’t use Internet services while you’re talking on the phone unless iPhone also has a Wi-Fi connection to the Internet. Depending on your network connection, you may not be able to receive calls while iPhone transfers data over the cellular network when downloading a webpage, for example. To learn more about the AT&T network, visit http://www.att.com/network.
To change cellular network settings, from the home screen, tap Settings.
To turn cellular data on or off, tap the Cellular Data switch. Note: To turn LTE data on or off tap Enable LTE > Desired option.
To turn data roaming on or off, tap the Data Roaming switch. Note: Data roaming is off by default. Turning on Data roaming will allow the device to attach to foreign wireless network signals when available. International roaming charges will apply. This setting does not apply to voice access, which requires additional provisioning for usage on international networks. To learn more, visit the AT&T International website.
To set whether cellular data is used for specific apps and services, scroll down then tap the Switch to the right of the Desired app.
To turn Wi-Fi Assist (automatically use cellular data when Wi-Fi connectivity is poor) on or off, scroll to the bottom of the page, then tap the Wi-Fi Assist switch. Note:Wi-Fi Assist regularly checks the Wi-Fi connection to determine signal strength. If the Wi-Fi signal strength drops below a specific range, Wi-Fi Assist will automatically switch the session to cellular data until the Wi-Fi signal improves. Wi-Fi Assist is an optional setting that is turned on by default and can be turned off at any time. Data rates apply for cellular connections. Learn more from Apple support article: About cellular data settings and usage on your iPhon