Apex domains cannot have CNAME records.
Question and response
Question
dig truecorp.co.th. NS
Answer
truecorp.co.th. 5 NS clarinet.asianet.co.th.
truecorp.co.th. 5 NS conductor.asianet.co.th.
Authority
The response has no authority records.
Additional
clarinet.asianet.co.th. 5 A 203.144.225.242
conductor.asianet.co.th. 5 A 203.144.255.72
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0