Apex domains cannot have CNAME records.
Question and response
Question
dig odontolaboral.edu.do. NS
Answer
odontolaboral.edu.do. 5 NS ns181224129104.a2dns.com.
odontolaboral.edu.do. 5 NS ns181224129103.a2dns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0