Apex domains cannot have CNAME records.
Question and response
Question
dig mastertel.com. NS
Answer
mastertel.com. 5 NS ns3.mastertel.ru.
mastertel.com. 5 NS ns1.mastertel.ru.
Authority
The response has no authority records.
Additional
ns1.mastertel.RU. 5 A 217.67.178.162
ns3.mastertel.RU. 5 A 178.238.117.124
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0