No available update
I already updated the software IOS 14.4 and no more update is available, and my phone started making calls and received massage but only for one day, and the error of the cellular failure starts appearing again and no network.
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Carrier Update (Required): Released by your Cellular Service provider (Carrier Update), affects your Outgoing / Incoming calls, SMS, Data etc..
How to Update Carrier Settings
You can manually check for and install a carrier settings, update with these steps: Make sure your device is connected to a Wi-Fi or cellular network. Tap Settings > General > About. If an update is available, you'll see an option to update your carrier settings.
Update your carrier settings on your iPhone or iPad - Apple Support