Apex domains cannot have CNAME records.
Question and response
Question
dig mobtelecom.com.br. NS
Answer
mobtelecom.com.br. 5 NS dns3.argohost.net.
mobtelecom.com.br. 5 NS dns2.argohost.net.
mobtelecom.com.br. 5 NS dns1.argohost.net.
Authority
The response has no authority records.
Additional
dns1.argohost.net. 5 A 201.20.117.54
dns2.argohost.net. 5 A 187.110.226.3
dns3.argohost.net. 5 A 187.110.226.2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0