Apex domains cannot have CNAME records.
Question and response
Question
dig ispio.com. NS
Answer
ispio.com. 3 NS ns2.ispio.com.
ispio.com. 3 NS ns1.nipa.co.th.
ispio.com. 3 NS ns6.nipa.co.th.
Authority
The response has no authority records.
Additional
ns2.ispio.com. 3 A 202.47.248.146
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0