Apex domains cannot have CNAME records.
Question and response
Question
dig online.com.kh. NS
Answer
online.com.kh. 5 NS dns1.online.com.kh.
online.com.kh. 5 NS dns7.online.com.kh.
online.com.kh. 5 NS dns5.online.com.kh.
online.com.kh. 5 NS dns4.online.com.kh.
online.com.kh. 5 NS dns3.online.com.kh.
online.com.kh. 5 NS dns2.online.com.kh.
Authority
The response has no authority records.
Additional
dns1.online.com.kh. 172691 A 203.189.128.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0