Apex domains cannot have CNAME records.
Question and response
Question
dig th. NS
Answer
th. 5 NS c.thains.co.th.
th. 5 NS p.thains.co.th.
th. 5 NS ns.thnic.net.
th. 5 NS nn1.thains.co.th.
th. 5 NS a.thains.co.th.
th. 5 NS b.thains.co.th.
Authority
The response has no authority records.
Additional
a.thains.co.th. 159539 A 122.155.23.64
b.thains.co.th. 139910 A 203.159.64.64
c.thains.co.th. 51206 A 194.0.1.28
p.thains.co.th. 144417 A 204.61.216.126
ns.thnic.net. 1 A 202.28.0.1
nn1.thains.co.th. 144417 A 194.146.106.154
a.thains.co.th. 159539 AAAA 2001:c38:2000:183:0:0:0:30
b.thains.co.th. 139910 AAAA 2405:3340:e011:3000:0:0:0:30
c.thains.co.th. 51206 AAAA 2001:678:4:0:0:0:0:1c
p.thains.co.th. 144417 AAAA 2001:500:14:6126:ad:0:0:1
nn1.thains.co.th. 144417 AAAA 2001:67c:1010:39:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0