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Does it say SIM Locked....or....does it say No SIM Restrictions?
If it says SIM Locked, you might have purchased an unlocked phone, but it is locked now to AT&T. Contact AT&T, but they may not be willing to help at this point based on the information that you have provided.
In other words, AT&T will not unlock the phone unless you are an AT&T customer.
If it says, No SIM Restrictions.....your next contact is to Mint Mobile for another try with a different SIM card. Things are up to Mint Mobile at this point.
Bottom line......Apple is not involved with your issue in any way.