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