Apex domains cannot have CNAME records.
Question and response
Question
dig test-cdn.GbLNet.RU. NS
Answer
test-cdn.GbLNet.RU. 5 NS ns2.GbLNet.RU.
test-cdn.GbLNet.RU. 5 NS ns.GbLNet.RU.
Authority
The response has no authority records.
Additional
ns.GbLNET.ru. 2 A 95.140.80.1
ns2.GbLNET.ru. 2 A 95.140.83.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0