Apex domains cannot have CNAME records.
Question and response
Question
dig gtsgroup.cz. NS
Answer
gtsgroup.cz. 5 NS hubserv.gts.cz.
gtsgroup.cz. 5 NS server1.gts.cz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0