Apex domains cannot have CNAME records.
Question and response
Question
dig wtnet.de. NS
Answer
wtnet.de. 5 NS dns3.wtnet.de.
wtnet.de. 5 NS dns2.wtnet.de.
wtnet.de. 5 NS dns1.wtnet.de.
Authority
The response has no authority records.
Additional
dns1.wtnet.de. 5 A 213.209.102.3
dns2.wtnet.de. 5 A 213.209.102.4
dns3.wtnet.de. 5 A 87.253.174.18
dns1.wtnet.de. 5 AAAA 2a02:2028:d000:0:0:0:0:3
dns2.wtnet.de. 5 AAAA 2a02:2028:d000:0:0:0:0:4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0