Apex domains cannot have CNAME records.
Question and response
Question
dig nrtmexico.mx. NS
Answer
nrtmexico.mx. 5 NS ns1.mexicosystem.com.
nrtmexico.mx. 5 NS ns2.mexicosystem.com.
Authority
The response has no authority records.
Additional
ns1.mexicosystem.com. 5 A 50.31.188.99
ns2.mexicosystem.com. 5 A 50.31.188.100
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0