Apex domains cannot have CNAME records.
Question and response
Question
dig CDN.GbLNET.ru. NS
Answer
cdn.gblnet.ru. 2 NS ns1.CDN.GbLNET.ru.
cdn.gblnet.ru. 2 NS ns.GbLNET.ru.
Authority
The response has no authority records.
Additional
ns.gblnet.ru. 2 A 95.140.80.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0