Apex domains cannot have CNAME records.
Question and response
Question
dig flexfonenet.dk. NS
Answer
flexfonenet.dk. 5 NS auth01.dns.sippeer.dk.
flexfonenet.dk. 5 NS auth02.dns.sippeer.dk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0