Apex domains cannot have CNAME records.
Question and response
Question
dig rt-rk.com. NS
Answer
rt-rk.com. 5 NS dns4.orion.rs.
rt-rk.com. 5 NS dns3.orion.rs.
rt-rk.com. 5 NS dns1.orion.rs.
rt-rk.com. 5 NS dns2.orion.rs.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0