Apex domains cannot have CNAME records.
Question and response
Question
dig uplus.co.kr. NS
Answer
uplus.co.kr. 4 NS ns2.lguplus.com.
uplus.co.kr. 4 NS ns3.lguplus.com.
uplus.co.kr. 4 NS ns1.lguplus.com.
Authority
The response has no authority records.
Additional
ns1.lguplus.com. 4 A 164.124.101.31
ns2.lguplus.com. 4 A 203.248.240.31
ns3.lguplus.com. 4 A 203.248.252.13
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0