Apex domains cannot have CNAME records.
Question and response
Question
dig schlundtech.de. NS
Answer
schlundtech.de. 5 NS any2.nodesecure.de.
schlundtech.de. 5 NS any3.nodesecure.eu.
schlundtech.de. 5 NS any4.nodesecure.info.
schlundtech.de. 5 NS any1.nodesecure.com.
Authority
The response has no authority records.
Additional
any2.nodesecure.de. 5 A 176.97.158.7
any3.nodesecure.eu. 5 A 185.132.35.251
any4.nodesecure.info. 5 A 192.174.68.90
any2.nodesecure.de. 5 AAAA 2001:67c:10b8:0:0:0:0:7
any3.nodesecure.eu. 5 AAAA 2607:f1c0:fe:53:185:132:35:251
any4.nodesecure.info. 5 AAAA 2001:67c:1bc:0:0:0:0:90
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0