Apex domains cannot have CNAME records.
Question and response
Question
dig 96.12.190.in-addr.arpa. NS
Answer
96.12.190.in-addr.arpa. 5 NS ns2.cps.com.ar.
96.12.190.in-addr.arpa. 5 NS ns1.cps.com.ar.
96.12.190.in-addr.arpa. 5 NS ns3.metrotel.ar.
Authority
The response has no authority records.
Additional
ns1.cps.com.ar. 5 A 190.12.96.200
ns2.cps.com.ar. 5 A 190.111.233.106
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0