Apex domains cannot have CNAME records.
Question and response
Question
dig netadmin.isl.net.tw. NS
Answer
netadmin.isl.net.tw. 1 NS ns2.isl.net.tw.
netadmin.isl.net.tw. 1 NS dns.isl.net.tw.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0