Apex domains cannot have CNAME records.
Question and response
Question
dig isp.ip.pt. NS
Answer
isp.ip.pt. 5 NS ns.ip.pt.
isp.ip.pt. 5 NS ns2.ip.pt.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0