Apex domains cannot have CNAME records.
Question and response
Question
dig mittwald.de. NS
Answer
mittwald.de. 5 NS ns01.agenturserver.de.
mittwald.de. 5 NS ns01.agenturserver.co.
mittwald.de. 5 NS ns01.agenturserver.it.
Authority
The response has no authority records.
Additional
ns01.agenturserver.co. 5 A 185.247.150.113
ns01.agenturserver.de. 5 A 185.247.151.111
ns01.agenturserver.it. 5 A 185.247.151.112
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0