Apex domains cannot have CNAME records.
Question and response
Question
dig cloud.mts.ru. NS
Answer
cloud.mts.ru. 5 NS cns.mws-mts.net.
cloud.mts.ru. 5 NS ens.mws.ru.
cloud.mts.ru. 5 NS ans.mws-mts.net.
cloud.mts.ru. 5 NS bns.mws.ru.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0