Apex domains cannot have CNAME records.
Question and response
Question
dig thai.com. NS
Answer
thai.com. 5 NS ns1.inet.co.th.
thai.com. 5 NS ns2.inet.co.th.
Authority
The response has no authority records.
Additional
ns1.inet.co.th. 5 A 172.64.52.6
ns1.inet.co.th. 5 A 172.64.53.197
ns2.inet.co.th. 5 A 172.64.52.115
ns2.inet.co.th. 5 A 172.64.53.220
ns1.inet.co.th. 5 AAAA 2606:4700:52:0:0:0:ac40:3406
ns1.inet.co.th. 5 AAAA 2606:4700:5a:0:0:0:ac40:35c5
ns2.inet.co.th. 5 AAAA 2606:4700:52:0:0:0:ac40:3473
ns2.inet.co.th. 5 AAAA 2606:4700:5a:0:0:0:ac40:35dc
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0