Apex domains cannot have CNAME records.
Question and response
Question
dig fibranetbrasil.net.br. NS
Answer
fibranetbrasil.net.br. 3 NS ns2.infortel.net.br.
fibranetbrasil.net.br. 3 NS ns1.infortel.net.br.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0