Apex domains cannot have CNAME records.
Question and response
Question
dig mts.ru. NS
Answer
mts.ru. 5 NS ns4.mts-ws.net.
mts.ru. 5 NS ns3.mts.ru.
mts.ru. 5 NS ns1.mts-ws.net.
mts.ru. 5 NS dns1.mtu.ru.
mts.ru. 5 NS ns2.mts.ru.
Authority
The response has no authority records.
Additional
ns1.mts-ws.net. 5 A 213.87.16.43
ns2.mts.ru. 5 A 213.87.50.254
ns3.mts.ru. 5 A 213.87.75.192
ns4.mts-ws.net. 5 A 217.8.236.228
dns1.mtu.ru. 5 A 212.188.8.37
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0