Apex domains cannot have CNAME records.
Question and response
Question
dig tot.co.th. NS
Answer
tot.co.th. 5 NS ns2.tot.co.th.
tot.co.th. 5 NS ns1.tot.co.th.
Authority
The response has no authority records.
Additional
ns2.tot.co.th. 5 A 203.113.1.13
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0