Apex domains cannot have CNAME records.
Question and response
Question
dig server-hotel-berlin.net. NS
Answer
server-hotel-berlin.net. 5 NS ns4.dns-hotel.net.
server-hotel-berlin.net. 5 NS ns3.dns-hotel.net.
server-hotel-berlin.net. 5 NS ns1.dns-hotel.net.
server-hotel-berlin.net. 5 NS ns2.dns-hotel.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0