Apex domains cannot have CNAME records.
Question and response
Question
dig 179.151.190.in-addr.arpa. NS
Answer
179.151.190.in-addr.arpa. 5 NS gcasrv.explora.com.sv.
Authority
The response has no authority records.
Additional
gcasrv.explora.com.sv. 5 A 190.151.191.2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0