Apex domains cannot have CNAME records.
Question and response
Question
dig alcanstelecom.com.br. NS
Answer
alcanstelecom.com.br. 3 NS ns1.alcans.com.br.
alcanstelecom.com.br. 3 NS ns2.alcans.com.br.
Authority
The response has no authority records.
Additional
ns1.alcans.com.br. 3 A 177.74.186.234
ns2.alcans.com.br. 3 A 177.74.186.235
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0