I got my first gen 12.9 from AT&T. After paying it off, and before attempting to sell it, I went to the AT&T website to try to unlock it, but kept getting messages that the unlocking failed. Then I checked and there was indeed an AT&T SIM card in the SIM card slot, meaning that AT&T doesn't use the internal SIM when you buy from them direct. Instead they must put their own SIM into the SIM card slot -- at least this was the case for the first gen 12.9.
So I simply removed that SIM, and then sold it. The new owner never complained about it being locked, so I assume all is well.
I haven't checked the SIM card slot in my new 10.5, which I also purchased from AT&T, but I assume it's the same.
If your 12.9 had an AT&T SIM card in the SIM card slot when you purchased it, then that must mean they did NOT lock the internal SIM to their network, since there would be no reason to use 2 SIMs.