Apex domains cannot have CNAME records.
Question and response
Question
dig static.rdcw.co.th. NS
Answer
static.rdcw.co.th. 5 NS ns1.srv.in.th.
static.rdcw.co.th. 5 NS ns2.srv.in.th.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0