Apex domains cannot have CNAME records.
Question and response
Question
dig isl.net.tw. NS
Answer
isl.net.tw. 4 NS dns.isl.net.tw.
isl.net.tw. 4 NS ns2.isl.net.tw.
Authority
The response has no authority records.
Additional
dns.isl.net.tw. 4 A 61.63.128.1
ns2.isl.net.tw. 4 A 61.63.137.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0