MQ8D2LL/A is it SIM free model?

Where did you buy the phone? It appears to be a Verizon Model, and the GSM side of all Verizon phones are unlocked. If you bought your phone anywhere other than directly from Apple, you didn't buy a sim free phone. Only Apple sells sim free phones.

If you're getting an invalid SIM message, then no. It is not an unlocked phone. The model number can not be used to determine whether or not a phone is unlocked.

Where did you buy the phone? The only source for unlocked iPhones in the US is directly from Apple. If you purchase the phone at Best Buy, or another retailer, the phone is NOT unlocked.

The only way to have the phone unlocked is to contact AT&T to ask if they will unlock the phone. Only the carrier the phone is locked to can authorize the unlock.

There is no trusted resource/website that can remove a lock.

If you are within the return window at Best Buy the best option is to return the phone and pay the restocking fee. Since the purchase was recent the extended holiday return period should apply.

AT&T does have a written unlock policy which you can consult online.

If you do not meet the criteria AT&T is under no obligation to unlock the phone.

There are cases where carriers have unlocked phones in such circumstances as yours but it is a hit or miss affair and pursuing that approach might foreclose the option to meet the return window at Best Buy.

The safest solution is to return the phone if you do not wish to use it on AT&T.

A similar problem. I sent the request to the operator (AT&T® Official - Unsupported Browser Page)

answer: approved

Use these instructions to unlock your device.

Unlock instructions:Contact Apple support if you have additional questions.

Thanks for choosing us, AT&T

iPhone still does not work

I tried full reset via iTunes. Does not help

Is this from Best Buy?

If so, it is not the sim free model as Best Buy does not sell it. It is subject to the US Resellers Flex Policy.