Apex domains cannot have CNAME records.
Question and response
Question
dig 79.21.196.in-addr.arpa. NS
Answer
79.21.196.in-addr.arpa. 4 NS ns1.tenet.ac.za.
79.21.196.in-addr.arpa. 4 NS ns2.tenet.ac.za.
79.21.196.in-addr.arpa. 4 NS ns3.tenet.ac.za.
79.21.196.in-addr.arpa. 4 NS ns4.tenet.ac.za.
Authority
The response has no authority records.
Additional
ns1.tenet.ac.za. 4 A 196.21.79.51
ns2.tenet.ac.za. 4 A 155.232.192.20
ns3.tenet.ac.za. 4 A 84.22.105.134
ns4.tenet.ac.za. 4 A 41.76.109.97
ns1.tenet.ac.za. 4 AAAA 2001:4200:ffff:a:0:0:0:51
ns2.tenet.ac.za. 4 AAAA 2001:4200:0:192a:0:0:0:20
ns3.tenet.ac.za. 4 AAAA 2a02:2770:17:0:21a:4aff:fe2e:48b9
ns4.tenet.ac.za. 4 AAAA 2c0f:f850:dc05:ea57:1:388c:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0