Apex domains cannot have CNAME records.
Question and response
Question
dig synaxon.de. NS
Answer
synaxon.de. 5 NS dns6.cloudns.net.
synaxon.de. 5 NS dns1.cloudns.net.
synaxon.de. 5 NS dns2.cloudns.net.
synaxon.de. 5 NS dns5.cloudns.net.
Authority
The response has no authority records.
Additional
dns2.cloudns.net. 5 A 185.136.97.77
dns2.cloudns.net. 5 AAAA 2a06:fb00:1:0:0:0:2:77
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0