Apex domains cannot have CNAME records.
Question and response
Question
dig avant.si. NS
Answer
avant.si. 5 NS ns1.avant.si.
avant.si. 5 NS ns3.avant.si.
avant.si. 5 NS ns4.avant.si.
avant.si. 5 NS ns0.avant.si.
Authority
The response has no authority records.
Additional
ns0.avant.si. 5 A 185.136.98.111
ns1.avant.si. 5 A 152.89.233.30
ns3.avant.si. 5 A 185.136.99.111
ns4.avant.si. 5 A 193.9.21.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0