Apex domains cannot have CNAME records.
Question and response
Question
dig resellerinterface.de. NS
Answer
resellerinterface.de. 2 NS ns2.mainns.eu.
resellerinterface.de. 2 NS ns3.mainns.net.
resellerinterface.de. 2 NS ns1.mainns.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0