Apex domains cannot have CNAME records.
Question and response
Question
dig designplus.cz. NS
Answer
designplus.cz. 5 NS diana.designplus.cz.
designplus.cz. 5 NS erin.designplus.cz.
Authority
The response has no authority records.
Additional
erin.designplus.cz. 5 A 185.156.123.113
diana.designplus.cz. 5 A 78.24.15.203
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0