Apex domains cannot have CNAME records.
Question and response
Question
dig 39.100.176.in-addr.arpa. NS
Answer
39.100.176.in-addr.arpa. 5 NS ns2.datalix.de.
39.100.176.in-addr.arpa. 5 NS ns1.datalix.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0