Apex domains cannot have CNAME records.
Question and response
Question
dig BITRIX24.RU. NS
Answer
BITRIX24.RU. 4 NS b.ns.selectel.RU.
BITRIX24.RU. 4 NS a.ns.selectel.RU.
BITRIX24.RU. 4 NS d.ns.selectel.RU.
BITRIX24.RU. 4 NS c.ns.selectel.RU.
Authority
The response has no authority records.
Additional
a.ns.selectel.ru. 4 A 188.68.203.1
b.ns.selectel.ru. 4 A 77.223.114.2
c.ns.selectel.ru. 4 A 188.68.203.3
d.ns.selectel.ru. 4 A 77.223.114.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0