Apex domains cannot have CNAME records.
Question and response
Question
dig customers.hiper-net.dk. NS
Answer
customers.hiper-net.dk. 4 NS ns4.hiper.dk.
customers.hiper-net.dk. 4 NS ns3.hiper.dk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0