Apex domains cannot have CNAME records.
Question and response
Question
dig tng.de. NS
Answer
tng.de. 4 NS ns1.ennit.net.
tng.de. 4 NS ns4.tng.de.
tng.de. 4 NS ns3.tng.de.
tng.de. 4 NS ns2.ennit.net.
Authority
The response has no authority records.
Additional
ns1.ennit.net. 4 A 213.178.66.161
ns2.ennit.net. 4 A 82.97.146.5
ns3.tng.de. 4 A 192.174.68.117
ns4.tng.de. 4 A 176.97.158.117
ns3.tng.de. 4 AAAA 2001:67c:1bc:0:0:0:0:117
ns4.tng.de. 4 AAAA 2001:67c:10b8:0:0:0:0:117
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0