What to do if iPhone 7 iOS 13 says SIM is invalid?

Question

iPhone

I bought a used phone, inserted my SIM card, checked - everything is in order. I used it for a year, tied iCloud, updated from 12 to 13 iOS, there were no problems. I decided to sell, the buyer inserted his SIM, checked everything, everything is fine. We started to do a reset, after which the activation of the sim took off. Says it's not valid. What could it be? Locked?
AT&T replied that this phone was not listed in their database.
IMEI: 359459080663243
Any other suggestions?

Answer 1

Assumption: it's in the SIM.
You need to check other sims for the fact that they work.

Answer 2

Locked.
Check Result:
Model: iPhone 7 32GB Black MM [A1660] [iPhone9,1]
IMEI: 359459080663243
Serial Number: F4GV2BYKHG6W
Purchase Date: 2017-09-27
Activation Status: Activated
Warranty Status: Out Of Warranty
Technical Support: Expired
Repairs and Service Coverage: Expired
Repairs and Service Expiration Date: 2018-09-26
Repairs and Service Expires In: (0) Days
Coverage End Date: 2018-09-26
Valid Purchase Date: Yes
AppleCare Eligible: No
Registered: Yes
Replaced: No
Loaner : No
Find My iPhone: ON
Carrier: US Sprint/Virgin/Boost (Sprint MVNO) Locked Activation Policy
SIMLock Status: Locked

Answer 3

It is necessary to untie through Apple if the contract period has expired. About 6 years ago, the British four was untied. Then no problem.