Apex domains cannot have CNAME records.
Question and response
Question
dig 129-253.217.0.81.in-addr.arpa. NS
Answer
129-253.217.0.81.in-addr.arpa. 5 NS ns.cybersales.cz.
129-253.217.0.81.in-addr.arpa. 5 NS sns.cybersales.cz.
Authority
The response has no authority records.
Additional
ns.cybersales.cz. 5 A 217.11.251.228
sns.cybersales.cz. 5 A 82.208.29.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0