Apex domains cannot have CNAME records.
Question and response
Question
dig 225-237.251.11.217.in-addr.arpa. NS
Answer
225-237.251.11.217.in-addr.arpa. 1 NS sns.cybersales.cz.
225-237.251.11.217.in-addr.arpa. 1 NS ns.cybersales.cz.
Authority
The response has no authority records.
Additional
ns.cybersales.cz. 1 A 217.11.251.228
sns.cybersales.cz. 1 A 82.208.29.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0