Apex domains cannot have CNAME records.
Question and response
Question
dig 131.252.185.rev.airmob.eu. NS
Answer
131.252.185.rev.airmob.eu. 5 NS rdns1.airmob.eu.
131.252.185.rev.airmob.eu. 5 NS rdns2.airmob.eu.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0