Apex domains cannot have CNAME records.
Question and response
Question
dig rumbo.com. NS
Answer
rumbo.com. 4 NS ns-460.awsdns-57.com.
rumbo.com. 4 NS ns-1694.awsdns-19.co.uk.
rumbo.com. 4 NS ns-1152.awsdns-16.org.
rumbo.com. 4 NS ns-637.awsdns-15.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0