Apex domains cannot have CNAME records.
Question and response
Question
dig CDN.GBLNet.RU. NS
Answer
cdn.gblnet.ru. 4 NS ns1.CDN.GBLNet.RU.
cdn.gblnet.ru. 4 NS ns.GBLNet.RU.
Authority
The response has no authority records.
Additional
ns.gblnet.RU. 4 A 95.140.80.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0