Jakemehometonight Said:
"iOS 15.4.1 update causing no sim with ATT: Updated my brand new iPhone 13. It isn’t connecting to ATT and says no sim. It was setup at the store with an eSim. I’ve done all the self help steps and guides, but I am still having no luck. This is already my second iPhone 13 in the first 2 weeks. What is happening!?"
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Troubleshoot "No SIM":
A. Have you Contacted AT&T?
I have seen plenty of reports on these forums for the last few weeks in regards to AT&T providing issues. And low and behold, the update has been round for that long. So, contact AT&T, see how they can fix this. They are you carrier, and they configure the SIM card. You'll find their contact us info on their Contact Us site of their domain.
B. "No SIM":
If "No SIM"*** is shown on the top-left corner of your screen, go here to fix that: If you See 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support
***Screenshot: