Apex domains cannot have CNAME records.
Question and response
Question
dig dynamdns.de. NS
Answer
dynamdns.de. 4 NS ns2.owe.de.
dynamdns.de. 4 NS ns.owe.de.
Authority
The response has no authority records.
Additional
ns.owe.de. 4 A 87.106.138.219
ns2.owe.de. 4 A 5.189.189.240
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0