Unlocking SM-T217S Galaxy Tab 3 Sprint. topic

I have the MSL code but when I put a sim card in for ATT and reset the device it says invalid sim card. I never get an option to enter the code in to unlock the device for another carrier. I am stock firmware not rooted. I actually tried loading ATT stock firmware on to the tablet and that didn't work at all. I can't take it to sprint because the person I bought it from is not in good standing with them.

Any help would be awesome!