Apex domains cannot have CNAME records.
Question and response
Question
dig nempost.dk. NS
Answer
nempost.dk. 4 NS ns.dandomain.dk.
nempost.dk. 4 NS ns2.dandomain.dk.
Authority
The response has no authority records.
Additional
ns.dandomain.dk. 1 A 185.25.141.3
ns2.dandomain.dk. 1 A 91.197.248.66
ns.dandomain.dk. 1 AAAA 2a02:2339:4006:109:0:0:0:dd
ns2.dandomain.dk. 1 AAAA 2a02:2339:1:108:0:0:dd:dd
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0