When Apple introduced the iPhone 12 series, the company added an option designed to improve your experience on your iPhone when you were on a Wi-Fi network, but your cellular plan gave you access to a higher-throughput 5G network. For instance, you can enable this option to have higher-quality FaceTime connections where using a Wi-Fi network would only provide lower-definition FaceTime. On an iPhone 12 model or later and iPads with cellular introduced after that, enabling Settings Cellular

Allow More Data on 5G lets your device make different choices. Some carriers may enable this option by default if you have an unlimited-usage plan. (That option might be under Settings
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