Apex domains cannot have CNAME records.
Question and response
Question
dig sdgtelecom.com.br. NS
Answer
sdgtelecom.com.br. 5 NS ns.sdgtelecom.com.br.
Authority
The response has no authority records.
Additional
ns.sdgtelecom.com.br. 5 A 45.236.12.6
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0