Apex domains cannot have CNAME records.
Question and response
Question
dig ampernet.com.br. NS
Answer
ampernet.com.br. 5 NS ns10.ampernet.com.br.
ampernet.com.br. 5 NS ns2.ampernet.com.br.
ampernet.com.br. 5 NS ns1.ampernet.com.br.
ampernet.com.br. 5 NS ns20.ampernet.com.br.
Authority
The response has no authority records.
Additional
ns10.ampernet.com.br. 5 A 187.60.210.23
ns20.ampernet.com.br. 5 A 187.60.210.5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0