Apex domains cannot have CNAME records.
Question and response
Question
dig assetplus.co.kr. NS
Answer
assetplus.co.kr. 5 NS ns4.siidc.net.
assetplus.co.kr. 5 NS ns3.siidc.net.
assetplus.co.kr. 5 NS ns1.siidc.net.
assetplus.co.kr. 5 NS ns5.siidc.net.
assetplus.co.kr. 5 NS ns2.siidc.net.
Authority
The response has no authority records.
Additional
ns1.siidc.net. 5 A 61.78.63.231
ns2.siidc.net. 5 A 210.112.124.126
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0