Apex domains cannot have CNAME records.
Question and response
Question
dig spb.avantel.ru. NS
Answer
spb.avantel.ru. 5 NS ns2.spb.avantel.ru.
spb.avantel.ru. 5 NS ns1.spb.avantel.ru.
Authority
The response has no authority records.
Additional
ns1.spb.avantel.ru. 5 A 188.68.187.70
ns2.spb.avantel.ru. 5 A 185.132.196.10
ns1.spb.avantel.ru. 5 AAAA 2a00:9801:11:1:0:0:0:11
ns2.spb.avantel.ru. 5 AAAA 2a00:9801:14:1:0:0:0:11
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0