Apex domains cannot have CNAME records.
Question and response
Question
dig nexgen.co.kr. NS
Answer
nexgen.co.kr. 1 NS ns1.hosting.co.kr.
nexgen.co.kr. 1 NS ns3.hosting.co.kr.
nexgen.co.kr. 1 NS ns2.hosting.co.kr.
nexgen.co.kr. 1 NS ns4.hosting.co.kr.
Authority
The response has no authority records.
Additional
ns1.hosting.co.kr. 1 A 121.254.170.11
ns2.hosting.co.kr. 1 A 43.201.141.93
ns3.hosting.co.kr. 1 A 121.254.170.12
ns4.hosting.co.kr. 1 A 43.201.141.93
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0