Apex domains cannot have CNAME records.
Question and response
Question
dig globaltechtelecom.com.br. NS
Answer
globaltechtelecom.com.br. 5 NS ns2.aveiro-mindnet.com.br.
globaltechtelecom.com.br. 5 NS ns1.aveiro-mindnet.com.br.
Authority
The response has no authority records.
Additional
ns1.aveiro-mindnet.com.br. 5 A 169.197.84.2
ns2.aveiro-mindnet.com.br. 5 A 169.197.84.3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0