Apex domains cannot have CNAME records.
Question and response
Question
dig 0-24.1.40.200.in-addr.arpa. NS
Answer
0-24.1.40.200.in-addr.arpa. 5 NS portal1.antel.com.uy.
0-24.1.40.200.in-addr.arpa. 5 NS portal2.antel.com.uy.
Authority
The response has no authority records.
Additional
portal1.antel.com.uy. 5 A 200.40.1.129
portal2.antel.com.uy. 5 A 200.40.1.193
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0