phone keep saying no sim then sos

Did you already have a look at theses troubleshooting tips?

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1. Make sure that you have an active plan with your wireless carrier. 
2. Restart your iPhone or iPad.
3. Check for a carrier settings update. Go to Settings > General > About. If an update is available, you'll see a prompt to select OK or Update.
4. If your device has an eSIM, contact your carrier.
5. If your device has a physical SIM card, remove your iPhone SIM card or iPad SIM card from the SIM card tray and then put the SIM card back. Make sure that the SIM tray closes completely and isn’t loose. If you use a SIM tray from a different model iPhone, iPad, or from another phone manufacturer, it might not fit properly.
6. If your device has a physical SIM card and you've completed the previous step, try using another SIM card. If you don't have one, go to your carrier's retail store and ask them to test with another SIM card. You might need to replace your SIM card.

If you still see an alert about your SIM card, contact Apple support.

Learn what to do if you see a SIM not supported alert.

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copied from: If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support

Did you try to switch back to 4G in the settings for Cellular?