Apex domains cannot have CNAME records.
Question and response
Question
dig thnic.co.th. NS
Answer
thnic.co.th. 5 NS e-ns.thnic.co.th.
thnic.co.th. 5 NS b-ns.thnic.co.th.
thnic.co.th. 5 NS a-ns.thnic.co.th.
thnic.co.th. 5 NS f-ns.thnic.co.th.
Authority
The response has no authority records.
Additional
a-ns.thnic.co.th. 5 A 103.155.66.46
b-ns.thnic.co.th. 5 A 103.206.25.46
e-ns.thnic.co.th. 5 A 194.58.198.136
f-ns.thnic.co.th. 5 A 185.42.137.136
a-ns.thnic.co.th. 5 AAAA 2001:df4:b880:0:0:0:0:46
b-ns.thnic.co.th. 5 AAAA 2001:df4:b881:0:0:0:0:46
e-ns.thnic.co.th. 5 AAAA 2a01:3f1:3032:8001:0:0:0:136
f-ns.thnic.co.th. 5 AAAA 2a01:3f0:400:8001:0:0:0:136
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0