Apex domains cannot have CNAME records.
Question and response
Question
dig nos.pt. NS
Answer
nos.pt. 5 NS ns6.nos.pt.
nos.pt. 5 NS ns5.nos.pt.
Authority
The response has no authority records.
Additional
ns5.nos.pt. 5 A 162.159.0.252
ns6.nos.pt. 5 A 162.159.1.254
ns5.nos.pt. 5 AAAA 2400:cb00:2049:1:0:0:a29f:fc
ns6.nos.pt. 5 AAAA 2400:cb00:2049:1:0:0:a29f:1fe
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0