It sounds like your phone is constantly searching for a cellular signal. I would use the following troubleshooting from the article named:
iPhone: Troubleshooting No Service
If you encounter either of the above symptoms on your iPhone, follow these steps to try to resolve the issue. Test after each step.
- Toggle airplane mode: Tap Settings > Enable Airplane Mode, wait five seconds, then turn off airplane mode.
- Turn your iPhone off, then on again.
- Remove the SIM card and verify that it's a valid, carrier-manufactured SIM. Also verify that it isn't damaged, worn, or modified. Then reinsert it.
- Check for a carrier-settings update. Connect to a Wi-Fi network. Then tap Settings > General > About. If an update is available, iOS will ask you if you want to install it. If Wi-Fi isn't available, connect your device to iTunes.
- Update your iPhone to the latest version of iOS.
- Reset network settings by tapping Settings > General > Reset > Reset Network Settings. This will reset all network settings, including Bluetooth pairing records, Wi-Fi passwords, VPN, and APN settings.
- Restore the iPhone.
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All the best,