Apex domains cannot have CNAME records.
Question and response
Question
dig isa.edu.do. NS
Answer
isa.edu.do. 5 NS ns2.dnsrubycom.com.
isa.edu.do. 5 NS ns3.dnsrubycom.com.
isa.edu.do. 5 NS ns4.dnsrubycom.com.
isa.edu.do. 5 NS ns1.dnsrubycom.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0