Apex domains cannot have CNAME records.
Question and response
Question
dig regiofiber.cz. NS
Answer
regiofiber.cz. 4 NS ans2.nsys.cz.
regiofiber.cz. 4 NS ans.nsys.cz.
Authority
The response has no authority records.
Additional
ans.nsys.cz. 4 A 89.190.92.132
ans2.nsys.cz. 4 A 37.157.197.117
ans.nsys.cz. 4 AAAA 2a00:bfe0:0:2:4488:1aff:fed7:c90f
ans2.nsys.cz. 4 AAAA 2a02:2b88:2:1:0:0:3018:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0