An update is required to use cellular data on this iPhone.

Hi iem145,

For help with updating your carrier settings, test the steps below found in the article here: Manually update your carrier settings on your iPhone or iPad

"If you dismiss the installation notification, you can manually update carrier settings by following these steps:

1. Make sure that your device is connected to Wi-Fi or a cellular network.
2. Tap Settings > General > About.
3. Follow the onscreen instructions to install your carrier update."

However, if you're seeing one of the specific messages listed here: If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart, follow the corresponding steps in the article. If the issue remains, reach out to Apple Support directly to further assist.

Contact - Official Apple Support

Contact Apple for support and service - Apple Support

Best.