Apex domains cannot have CNAME records.
Question and response
Question
dig UNISENDER.RU. NS
Answer
unisender.ru. 5 NS ns2.yandexcloud.net.
unisender.ru. 5 NS ns1.yandexcloud.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0