Apex domains cannot have CNAME records.
Question and response
Question
dig boreus.de. NS
Answer
boreus.de. 5 NS ns2.boreus.de.
boreus.de. 5 NS ns4.boreus.de.
boreus.de. 5 NS ns1.boreus.de.
boreus.de. 5 NS ns3.boreus.de.
Authority
The response has no authority records.
Additional
ns1.boreus.de. 5 A 208.82.72.7
ns2.boreus.de. 5 A 185.21.124.250
ns3.boreus.de. 5 A 185.21.125.2
ns4.boreus.de. 5 A 18.196.15.34
ns4.boreus.de. 5 AAAA 2a05:d014:5c9:5a00:10ed:d525:2691:834e
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0