Apex domains cannot have CNAME records.
Question and response
Question
dig 49.20.81.in-addr.arpa. NS
Answer
49.20.81.in-addr.arpa. 5 NS ns2.as39202.co.uk.
49.20.81.in-addr.arpa. 5 NS ns1.as39202.net.
49.20.81.in-addr.arpa. 5 NS ns0.as39202.net.
Authority
The response has no authority records.
Additional
ns0.as39202.net. 5 A 81.20.49.93
ns1.as39202.net. 5 A 81.20.49.60
ns2.as39202.co.uk. 5 A 81.20.48.93
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0