Apex domains cannot have CNAME records.
Question and response
Question
dig modem.dynamic.cptelecom.net. NS
Answer
modem.dynamic.cptelecom.net. 4 NS ns.cpinternet.com.
modem.dynamic.cptelecom.net. 4 NS ns2.cpinternet.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0