Apex domains cannot have CNAME records.
Question and response
Question
dig att.com. NS
Answer
att.com. 5 NS a11-65.akam.net.
att.com. 5 NS a5-66.akam.net.
att.com. 5 NS a2-64.akam.net.
att.com. 5 NS a4-64.akam.net.
att.com. 5 NS a1-187.akam.net.
att.com. 5 NS a22-67.akam.net.
Authority
The response has no authority records.
Additional
a22-67.akam.net. 5 A 23.211.61.67
a22-67.akam.net. 5 AAAA 2600:1480:7800:0:0:0:0:43
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0