Apex domains cannot have CNAME records.
Question and response
Question
dig media4web.cz. NS
Answer
media4web.cz. 5 NS dns.media4web.cz.
media4web.cz. 5 NS sns.media4web.cz.
Authority
The response has no authority records.
Additional
dns.media4web.cz. 5 A 81.0.235.1
sns.media4web.cz. 5 A 93.185.100.244
dns.media4web.cz. 5 AAAA 2001:1528:132:0:0:0:0:60
sns.media4web.cz. 5 AAAA 2a00:1ed0:46:200:0:0:1212:247
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0