Apex domains cannot have CNAME records.
Question and response
Question
dig providerbox.de. NS
Answer
providerbox.de. 5 NS dns01.providerbox.de.
providerbox.de. 5 NS dns02.providerbox.de.
providerbox.de. 5 NS dns03.providerbox.de.
Authority
The response has no authority records.
Additional
dns01.providerbox.de. 5 A 109.237.142.254
dns02.providerbox.de. 5 A 148.251.254.106
dns03.providerbox.de. 5 A 109.237.143.126
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0