Apex domains cannot have CNAME records.
Question and response
Question
dig 158.51.in-addr.arpa. NS
Answer
158.51.in-addr.arpa. 4 NS nsb.online.net.
158.51.in-addr.arpa. 4 NS nsa.online.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0