Apex domains cannot have CNAME records.
Question and response
Question
dig MANGO-OFFICE.RU. NS
Answer
MANGO-OFFICE.RU. 1 NS nss2.mango-office.ru.
MANGO-OFFICE.RU. 1 NS ns4-l2.nic.ru.
MANGO-OFFICE.RU. 1 NS ns8-l2.nic.ru.
MANGO-OFFICE.RU. 1 NS nss1.mango-office.ru.
Authority
The response has no authority records.
Additional
nss1.MANGO-OFFICE.RU. 1 A 178.248.232.131
nss2.MANGO-OFFICE.RU. 1 A 178.248.236.128
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0