Asked at the counter: “My iPhone keeps saying ‘No SIM’ or ‘Invalid SIM’ and I lose signal.

Apple’s own sequence for this alert, in order. None of it affects your data.

FREE, ONE-MINUTE CHECKS

• Restart the iPhone.
• Check for a network provider settings update: Settings, General, About. If an update is waiting, a prompt appears after a few seconds.

SIMPLE THINGS TO TRY AT HOME

• Switch the phone off. Open the SIM tray with the tool that came with the phone or a straightened paperclip, pushed gently into the small hole on the tray (not the microphone hole next to it on some models).
• Take the SIM out, check it for scratches or damage, wipe the gold contacts with a dry cloth, and put it back the right way round; it only fits one way.
• Apple’s specific check: make sure the tray closes completely and is not loose. A tray that does not sit flush, or the wrong tray from another phone, causes intermittent No SIM.
• Try another SIM card if you have one. If you do not, Apple suggests asking your network provider’s shop to test with one of theirs. If another SIM works, the fault is your card, and the network will replace it, normally free.
• If your phone uses an eSIM, contact the network; there is nothing to reseat.

WHEN IT PROBABLY NEEDS A REPAIR BUSINESS

• Two different SIMs both show No SIM in this phone but work in another.
• The tray does not sit flush, is bent, or the phone has been dropped on that corner or been wet.
• Signal drops whenever the phone flexes slightly in your hand.

That points to the SIM reader or its connection, which a business can test in minutes.

Sources checked: Apple Support (UK): If you see Invalid SIM or No SIM on your iPhone or iPad.