Apex domains cannot have CNAME records.
Question and response
Question
dig return-path.dk. NS
Answer
return-path.dk. 2 NS ns2.simply.com.
return-path.dk. 2 NS ns3.simply.com.
return-path.dk. 2 NS ns1.simply.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0