Apex domains cannot have CNAME records.
Question and response
Question
dig mailmasterplus.net. NS
Answer
mailmasterplus.net. 4 NS ns-1719.awsdns-22.co.uk.
mailmasterplus.net. 4 NS ns-319.awsdns-39.com.
mailmasterplus.net. 4 NS ns-535.awsdns-02.net.
mailmasterplus.net. 4 NS ns-1526.awsdns-62.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0