Apex domains cannot have CNAME records.
Question and response
Question
dig rdb.idcfcloud.net. NS
Answer
rdb.idcfcloud.net. 2 NS ns01.idcfcloud.com.
rdb.idcfcloud.net. 2 NS ns03.idcfcloud.com.
rdb.idcfcloud.net. 2 NS ns02.idcfcloud.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0