Apex domains cannot have CNAME records.
Question and response
Question
dig ismt.pt. NS
Answer
ismt.pt. 5 NS ns-1.ismt.pt.
ismt.pt. 5 NS ns-2.ismt.pt.
Authority
The response has no authority records.
Additional
ns-1.ismt.pt. 5 A 62.28.187.145
ns-2.ismt.pt. 5 A 62.28.187.146
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0