Why always showing no service in cellular data?

See these Apple articles below:

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

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

Manually update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

Update your network provider settings manually on your iPhone or iPad

Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)

Read again ->Related Article: If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart

Another possible fix is:

Remove and reinsert the SIM card.

If it doesn't help contact your carrier, maybe the SIM card is damaged, ask for a replaced (new) card. Normally it is free.

Also ask to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.

emtiaj ~ What model of iPhone is it?...

Settings > General > About > Model Name

And what version of iOS is it running?...

Settings > General > About > Software Version

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