Why does cellular issue keep happening

The link on top of this page which you had visited, before posting this question here, is shown under the "Related Article” that may have given you an answer.

If you are unable make a call or receive; If you can't make or receive calls on your iPhone - Apple Support

Cellular Update Failed —> If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart

Settings > Mobile Data > APN [under Mobile Data APN] enter the APN pin

View and edit your APN settings

If your carrier allows it, you can view your APN settings in one of the following locations: Settings > Cellular > Cellular Data Options > Cellular Network. Settings> Mobile Data > Mobile Data Options > Mobile Data Network.

View and edit your Access Point Name (APN) on your iPhone ...

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

If nothing helps then --> If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart