Apex domains cannot have CNAME records.
Question and response
Question
dig x2.si. NS
Answer
x2.si. 5 NS x2.si.
x2.si. 5 NS x1.si.
Authority
The response has no authority records.
Additional
x1.si. 5 A 217.61.252.153
x2.si. 5 A 185.185.91.53
x1.si. 5 AAAA 2a02:28b7:5301:0:0:0:0:53
x2.si. 5 AAAA 2a02:28b7:5302:0:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0