Apex domains cannot have CNAME records.
Question and response
Question
dig t-ipconnect.de. NS
Answer
t-ipconnect.de. 5 NS pns.dtag.de.
t-ipconnect.de. 5 NS dns20.dns.t-ipnet.de.
t-ipconnect.de. 5 NS dns02.dns.t-ipnet.de.
t-ipconnect.de. 5 NS dns00.dns.t-ipnet.de.
t-ipconnect.de. 5 NS dns50.dns.t-ipnet.de.
Authority
The response has no authority records.
Additional
pns.dtag.de. 4 A 194.25.0.125
dns00.dns.t-ipnet.de. 5 A 194.25.2.170
dns02.dns.t-ipnet.de. 5 A 194.25.2.172
dns20.dns.t-ipnet.de. 5 A 212.185.24.129
dns50.dns.t-ipnet.de. 5 A 217.5.100.183
pns.dtag.de. 4 AAAA 2003:40:8000:0:0:0:0:100
dns00.dns.t-ipnet.de. 5 AAAA 2003:180:4:a:0:0:0:53
dns02.dns.t-ipnet.de. 5 AAAA 2003:180:4:a:0:0:1:53
dns20.dns.t-ipnet.de. 5 AAAA 2003:180:a:2000:0:0:0:53
dns50.dns.t-ipnet.de. 5 AAAA 2003:180:4:10a:0:0:2:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0