Apex domains cannot have CNAME records.
Question and response
Question
dig dip0.t-ipconnect.de. NS
Answer
dip0.t-ipconnect.de. 5 NS dns40.dns.t-ipnet.de.
dip0.t-ipconnect.de. 5 NS dns04.dns.t-ipnet.de.
dip0.t-ipconnect.de. 5 NS dns01.dns.t-ipnet.de.
dip0.t-ipconnect.de. 5 NS dns51.dns.t-ipnet.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0