Nighthawk MR1100 AT&T Unlock Help

Good evening,

      I need help unlocking this device for use with other networks.  It is currently locked to AT&Ts network.  I purchased it outright and have no balance remaining.  

      I have already sent a request to AT&T for an unlock code.  They provided me with an 8 digit pin with no instructions.  I called and talked to tech support yesterday for an hour.  The only thing that came out of it was that I need a sim card from a different carrier to be able to unlock the device.  I do not have that and also am curious if that is true. 

      I need to unlock the device to help me sell the thing.  I no longer need it and not everyone has AT&T.  Is it true that the only way to unlock the device is to input the code with a different sim installed?  What are the odds that a potential buyer is already going to have a spare sim to use the code on?  On top of that, is the code a one time unlock for that card only?  And if the owner decides to change carriers they will have to go through all of this again?

      If someone could explain this and possibly post instructions, it would be greatly appreciated.

Thank you,

Chris