A:AnswerPossibly. It will work no problem if the device is network unlocked. If the device is network locked, it will only work if the network it is locked to is AT&T, or an AT&T affiliate.
A:AnswerYes. An ATT locked phone will work on any carrier/MVNO that uses ATT's network. *I am not an AT&T employee, but I have put a Cricket SIM cards in att locked pixel 5, among other devices, and it worked.