Apex domains cannot have CNAME records.
Question and response
Question
dig smilexd.co.th. NS
Answer
smilexd.co.th. 5 NS seth.ns.cloudflare.com.
smilexd.co.th. 5 NS joan.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0