Apex domains cannot have CNAME records.
Question and response
Question
dig isp.telekom.rs. NS
Answer
isp.telekom.rs. 5 NS dagda2.isp.telekom.rs.
isp.telekom.rs. 5 NS dagda1.isp.telekom.rs.
Authority
The response has no authority records.
Additional
dagda1.isp.telekom.rs. 5 A 212.200.191.150
dagda2.isp.telekom.rs. 5 A 212.200.190.150
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0