Apex domains cannot have CNAME records.
Question and response
Question
dig thains.co.th. NS
Answer
thains.co.th. 5 NS nn.thains.co.th.
thains.co.th. 5 NS ns2.thains.co.th.
Authority
The response has no authority records.
Additional
nn.thains.co.th. 5 A 194.58.198.32
ns2.thains.co.th. 5 A 203.159.64.65
nn.thains.co.th. 5 AAAA 2a01:3f1:3032:0:0:0:0:53
ns2.thains.co.th. 5 AAAA 2405:3340:e011:3000:0:0:0:31
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0