Apex domains cannot have CNAME records.
Question and response
Question
dig spf.ais.co.th. NS
Answer
spf.ais.co.th. 5 NS ns3.ais.co.th.
spf.ais.co.th. 5 NS ns1.ais-idc.com.
spf.ais.co.th. 5 NS ns2.ais-idc.com.
spf.ais.co.th. 5 NS ns4.ais.co.th.
Authority
The response has no authority records.
Additional
ns1.ais-idc.com. 4 A 49.231.33.18
ns2.ais-idc.com. 4 A 49.231.33.154
ns3.ais.co.th. 4 A 110.49.131.245
ns4.ais.co.th. 4 A 110.49.135.245
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0