Apex domains cannot have CNAME records.
Question and response
Question
dig stadtwerke-soest.de. NS
Answer
stadtwerke-soest.de. 1 NS ns02.stadtwerke-soest.net.
stadtwerke-soest.de. 1 NS ns03.stadtwerke-soest.net.
stadtwerke-soest.de. 1 NS ns01.stadtwerke-soest.net.
Authority
The response has no authority records.
Additional
ns01.stadtwerke-soest.net. 1 A 185.248.191.2
ns02.stadtwerke-soest.net. 1 A 185.248.188.2
ns03.stadtwerke-soest.net. 1 A 51.68.177.194
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0