Apex domains cannot have CNAME records.
Question and response
Question
dig yettel.rs. NS
Answer
yettel.rs. 5 NS dns2.cetin.rs.
yettel.rs. 5 NS dinosaur.cetin.rs.
yettel.rs. 5 NS ns2.cetin.rs.
yettel.rs. 5 NS dnssrv1.cloudhosting.rs.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0