Chosen Solution

My 6S stopped working after upgrading to iOS 12. Currently, it will operate normally WITHOUT the SIM card inserted. But when a SIM card is inserted, a notification pops said “Activation Required”. Tap “Dismiss”, and it will continue to “Hello” screen (like the first time it turned on), a moment later it’ll switch to home screen, without any cell reception, and pops another “Activation Required”, and repeat the whole thing mentioned. I tried reverting to iOS 11.4, but as I write this, it still no result.

You may want to speak with your cellular service provider about this situation. I doubt it is hardware related so I think that might be your best option at the moment.