Apex domains cannot have CNAME records.
Question and response
Question
dig fibron.com.br. NS
Answer
fibron.com.br. 4 NS e.sec.dns.br.
fibron.com.br. 4 NS d.sec.dns.br.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0