Apex domains cannot have CNAME records.
Question and response
Question
dig uns.AC.RS. NS
Answer
uns.AC.RS. 4 NS ns.rcub.bg.AC.RS.
uns.AC.RS. 4 NS ns2.uns.AC.RS.
uns.AC.RS. 4 NS ns1.uns.AC.RS.
Authority
The response has no authority records.
Additional
ns.rcub.bg.ac.rs. 4 A 147.91.1.5
ns1.uns.ac.rs. 4 A 147.91.173.4
ns2.uns.ac.rs. 4 A 147.91.173.5
ns.rcub.bg.ac.rs. 4 AAAA 2001:4170:0:1:0:0:0:5
ns1.uns.ac.rs. 4 AAAA 2001:4170:2000:1:0:0:0:4
ns2.uns.ac.rs. 4 AAAA 2001:4170:2000:1:0:0:0:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0