Apex domains cannot have CNAME records.
Question and response
Question
dig zubrodev.cz. NS
Answer
zubrodev.cz. 5 NS ns2.web4u.cz.
zubrodev.cz. 5 NS ns.web4u.cz.
Authority
The response has no authority records.
Additional
ns.web4u.cz. 5 A 185.14.224.241
ns2.web4u.cz. 5 A 85.207.53.12
ns.web4u.cz. 5 AAAA 2001:1568:fd:0:0:0:0:241
ns2.web4u.cz. 5 AAAA 2a02:a40:100:28ff:0:0:0:12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0