Apex domains cannot have CNAME records.
Question and response
Question
dig hk.dyxnet.com. NS
Answer
hk.dyxnet.com. 2 NS ns2.diyixian.com.
hk.dyxnet.com. 2 NS ns1.diyixian.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0