Apex domains cannot have CNAME records.
Question and response
Question
dig mioweb-hosting.cz. NS
Answer
mioweb-hosting.cz. 5 NS ns-1417.awsdns-49.org.
mioweb-hosting.cz. 5 NS ns-873.awsdns-45.net.
mioweb-hosting.cz. 5 NS ns-33.awsdns-04.com.
mioweb-hosting.cz. 5 NS ns-1768.awsdns-29.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0