Apex domains cannot have CNAME records.
Question and response
Question
dig smartclick.co.th. NS
Answer
smartclick.co.th. 5 NS ns7.smartclickonline.com.
smartclick.co.th. 5 NS ns6.smartclickonline.com.
Authority
The response has no authority records.
Additional
ns6.smartclickonline.com. 5 A 203.146.102.39
ns7.smartclickonline.com. 5 A 203.146.102.39
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0