Apex domains cannot have CNAME records.
Question and response
Question
dig 203.241.162.in-addr.arpa. NS
Answer
203.241.162.in-addr.arpa. 5 NS ns1.unifiedlayer.com.
203.241.162.in-addr.arpa. 5 NS ns2.unifiedlayer.com.
Authority
The response has no authority records.
Additional
ns1.unifiedlayer.com. 2 A 162.159.24.11
ns2.unifiedlayer.com. 2 A 162.159.25.92
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0