I reset my iPhone 6 my daughter gave me and it works great but when I put in a new AT
Why won’t my AT&T SIM card work in my iPhone 6.
iPhone 6, iOS 11.2.6
Most likely, the previous SIM card is "locked" to a cellular provider. You will need to contact that provider to "unlock" the iPhone before you can use a new SIM card with it.
- Wireless carrier support and features for iPhone - Apple Support
- If you can't connect to a cellular network or cellular data - Apple Support
- If you get an alert about the SIM card in your iPhone - Apple Support
- 7 Things To Do When Switching iPhone Carriers- Lifewire