Apex domains cannot have CNAME records.
Question and response
Question
dig 192-255.50.106.59.in-addr.arpa. NS
Answer
192-255.50.106.59.in-addr.arpa. 2 NS dns.nnetworks.jp.
192-255.50.106.59.in-addr.arpa. 2 NS dns.nnetworks.co.jp.
Authority
The response has no authority records.
Additional
dns.nnetworks.co.jp. 2 A 110.232.195.139
dns.nnetworks.jp. 2 A 59.106.50.252
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0