Apex domains cannot have CNAME records.
Question and response
Question
dig onlh.de. NS
Answer
onlh.de. 5 NS ns2.as213579.net.
onlh.de. 5 NS ns1.onlh.de.
Authority
The response has no authority records.
Additional
ns1.onlh.de. 5 A 185.140.54.7
ns2.as213579.net. 5 A 152.53.131.121
ns1.onlh.de. 5 AAAA 2a14:47c0:e047:0:0:0:0:7
ns2.as213579.net. 5 AAAA 2a0a:4cc0:c0:cd:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0