Apex domains cannot have CNAME records.
Question and response
Question
dig ptr83.nosmadeira.net. NS
Answer
ptr83.nosmadeira.net. 5 NS ns1.nosmadeira.com.
ptr83.nosmadeira.net. 5 NS ns2.nosmadeira.com.
Authority
The response has no authority records.
Additional
ns1.nosmadeira.com. 5 A 213.190.197.4
ns2.nosmadeira.com. 5 A 213.190.196.5
ns1.nosmadeira.com. 5 AAAA 2a00:d180:8004:4:0:0:197:4
ns2.nosmadeira.com. 5 AAAA 2a00:d180:8004:6:0:0:196:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0