I've tried on 3 different Surface 3 LTE models and I honestly don't think you can unlock them. AT&T has stated at different times that tablets are not locked (this one is) and that Microsoft will have to take care of it. Or that Microsoft won't share the unlock codes with them. I've been round and round since December and 2 replacements and none can be unlocked. The last Microsoft tech told me that it was considered tampering, was illegal and would void my warranty even if they could.
I have would suggest never buying an AT&T branded tablet of any type.
I've tried on 3 different Surface 3 LTE models and I honestly don't think you can unlock them. AT&T has stated at different times that tablets are not locked (this one is) and that Microsoft will have to take care of it.
As I stated before, I had 2 asus AT&T branded tablets, I just went into system information to get the IMEI, and you enter that into AT&Ts unlock site, and you get your code, Once you install another companies sim, it will give the option to enter your code. Worked for me.