Apex domains cannot have CNAME records.
Question and response
Question
dig convex.ru. NS
Answer
convex.ru. 5 NS dns2.convex.ru.
convex.ru. 5 NS dns.convex.ru.
Authority
The response has no authority records.
Additional
dns.convex.ru. 5 A 79.172.16.1
dns2.convex.ru. 5 A 95.183.42.3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0