Apex domains cannot have CNAME records.
Question and response
Question
dig sepe.dk. NS
Answer
sepe.dk. 1 NS ns4.simply.com.
sepe.dk. 1 NS ns2.simply.com.
sepe.dk. 1 NS ns1.simply.com.
sepe.dk. 1 NS ns3.simply.com.
Authority
The response has no authority records.
Additional
ns1.simply.com. 1 A 91.197.248.78
ns2.simply.com. 1 A 185.25.141.15
ns3.simply.com. 1 A 91.197.248.79
ns1.simply.com. 1 AAAA 2a02:2339:1:15c:0:0:4562:4562
ns2.simply.com. 1 AAAA 2a02:2339:4006:101:0:0:4562:4562
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0