Doesn't catch cellular communication

Try to optimize the range of the mobile network, see this Apple article:

If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

You need to contact your carrier for help with this. Make sure you have a SIM card from them and that your account is set up correctly.

My phone is not tied to a cellular operator.

Then you will have no service.

I'm in another country now and can't find online support to check

You use a local carrier, do you?

If they are On, turn Off Hotspot & VPN.

If your carrier confirms that there's no issue with your account or network, but you still can't connect to a cellular network, contact Apple Support.

See also this Apple article: Use captive Wi-Fi networks on your iPhone, iPad or iPod touch

Use captive Wi-Fi networks on your iPhone, iPad or iPod touch – Apple Support (UK)

Have you contact your carrier?

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