Apex domains cannot have CNAME records.
Question and response
Question
dig gtm.oss-core.net. NS
Answer
gtm.oss-core.net. 5 NS sydcoregtm01.gtm.oss-core.net.
gtm.oss-core.net. 5 NS melcoregtm01.gtm.oss-core.net.
Authority
The response has no authority records.
Additional
melcoregtm01.gtm.oss-core.net. 5 A 203.134.65.68
sydcoregtm01.gtm.oss-core.net. 5 A 203.134.64.68
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0