Apex domains cannot have CNAME records.
Question and response
Question
dig tf.uns.ac.rs. NS
Answer
tf.uns.ac.rs. 5 NS tehnol.tf.uns.ac.rs.
tf.uns.ac.rs. 5 NS ns1.uns.ac.rs.
Authority
The response has no authority records.
Additional
ns1.uns.ac.rs. 3 A 147.91.173.4
tehnol.tf.uns.ac.rs. 5 A 147.91.172.49
ns1.uns.ac.rs. 3 AAAA 2001:4170:2000:1:0:0:0:4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0