We received a new iPhone under warrantee and are not able to set it up

we received a new iPhone under warrantee and are not able to set it up. The message on the phone says:

SIM card is from a carrier that is not supported.

our Cartier is AT&T and I even took it in for a new SIM card but that did not fix the issue. What needs to happen to fix this issue?

Serial number of original product 

FF***XKN

Serial number of replacement product 

FF***XKN 

[Edited by Moderator]