Apex domains cannot have CNAME records.
Question and response
Question
dig rtl2.de. NS
Answer
rtl2.de. 5 NS ns1.cc-net.de.
rtl2.de. 5 NS pfire-1.rtl2.de.
rtl2.de. 5 NS ns2.cc-net.de.
rtl2.de. 5 NS ns1.rtl2.de.
rtl2.de. 5 NS pfire-2.rtl2.de.
Authority
The response has no authority records.
Additional
ns1.rtl2.de. 5 A 185.181.224.4
ns1.cc-net.de. 5 A 213.183.80.5
ns2.cc-net.de. 5 A 213.183.81.5
ns1.rtl2.de. 5 AAAA 2a0a:d480:1:1:0:0:0:100
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0