Apex domains cannot have CNAME records.
Question and response
Question
dig dts-online.net. NS
Answer
dts-online.net. 5 NS ns3.icsmedia.de.
dts-online.net. 5 NS ns1.dts-online.net.
dts-online.net. 5 NS ns2.dts-online.net.
Authority
The response has no authority records.
Additional
ns1.dts-online.net. 5 A 84.201.73.35
ns2.dts-online.net. 5 A 212.62.68.35
ns3.icsmedia.de. 5 A 212.101.192.10
ns1.dts-online.net. 5 AAAA 2a00:1348:0:1:23:0:0:1
ns2.dts-online.net. 5 AAAA 2a00:1348:0:0:23:0:0:1
ns3.icsmedia.de. 5 AAAA 2a06:7880:1:1:53:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0