Apex domains cannot have CNAME records.
Question and response
Question
dig ns2-tec.de. NS
Answer
ns2-tec.de. 5 NS ns2-tec.de.
ns2-tec.de. 5 NS ns1-tec.de.
Authority
The response has no authority records.
Additional
ns1-tec.de. 5 A 45.67.68.12
ns2-tec.de. 5 A 92.246.85.245
ns1-tec.de. 5 AAAA 2a09:71c0:0:0:0:101:0:12
ns2-tec.de. 5 AAAA 2a0d:c2c0:1:d:0:0:0:12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0