Apex domains cannot have CNAME records.
Question and response
Question
dig dnspro.pt. NS
Answer
dnspro.pt. 5 NS ns1.dnspro.pt.
dnspro.pt. 5 NS ns2.dnspro.pt.
dnspro.pt. 5 NS ns3.dnspro.pt.
dnspro.pt. 5 NS ns4.dnspro.pt.
Authority
The response has no authority records.
Additional
ns1.dnspro.pt. 5 A 185.136.96.168
ns2.dnspro.pt. 5 A 185.136.97.168
ns3.dnspro.pt. 5 A 185.136.98.168
ns4.dnspro.pt. 5 A 185.136.99.168
ns1.dnspro.pt. 5 AAAA 2a06:fb00:1:0:0:0:1:168
ns2.dnspro.pt. 5 AAAA 2a06:fb00:1:0:0:0:2:168
ns3.dnspro.pt. 5 AAAA 2a06:fb00:1:0:0:0:3:168
ns4.dnspro.pt. 5 AAAA 2a06:fb00:1:0:0:0:4:168
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0