Apex domains cannot have CNAME records.
Question and response
Question
dig 183.72.131.in-addr.arpa. NS
Answer
183.72.131.in-addr.arpa. 5 NS ns1.computex.net.br.
183.72.131.in-addr.arpa. 5 NS ns2.computex.net.br.
Authority
The response has no authority records.
Additional
ns1.computex.net.br. 5 A 131.72.183.200
ns2.computex.net.br. 5 A 131.72.183.210
ns1.computex.net.br. 5 AAAA 2804:1b9c:400:0:0:0:0:2
ns2.computex.net.br. 5 AAAA 2804:1b9c:400:0:0:0:0:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0