Apex domains cannot have CNAME records.
Question and response
Question
dig online.net.br. NS
Answer
online.net.br. 2 NS ns01.online.net.br.
online.net.br. 2 NS ns2.online.net.br.
Authority
The response has no authority records.
Additional
ns2.online.net.br. 2 A 191.7.215.18
ns01.online.net.br. 2 A 191.7.213.226
ns2.online.net.br. 2 AAAA 2804:145c:0:b:0:0:0:1
ns01.online.net.br. 2 AAAA 2804:145c:0:4:0:0:0:12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0