Apex domains cannot have CNAME records.
Question and response
Question
dig telnor.net. NS
Answer
telnor.net. 5 NS nsa-bcn-piopico-1.uninet.net.mx.
telnor.net. 5 NS dnsadm-interno.uninet.net.mx.
telnor.net. 5 NS nsgdl1.uninet.net.mx.
telnor.net. 5 NS nsa-bcn-arbol-1.uninet.net.mx.
Authority
The response has no authority records.
Additional
nsgdl1.uninet.net.mx. 5 A 200.23.242.197
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0