Apex domains cannot have CNAME records.
Question and response
Question
dig dynamic.isl.net.tw. NS
Answer
dynamic.isl.net.tw. 5 NS dns.isl.net.tw.
dynamic.isl.net.tw. 5 NS ns2.isl.net.tw.
Authority
The response has no authority records.
Additional
dns.isl.net.tw. 2 A 61.63.128.1
ns2.isl.net.tw. 2 A 61.63.137.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0