Apex domains cannot have CNAME records.
Question and response
Question
dig ip4.static.sl-reverse.com. NS
Answer
ip4.static.sl-reverse.com. 5 NS ns1.arpa.networklayer.com.
ip4.static.sl-reverse.com. 5 NS ns2.arpa.networklayer.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0