Apex domains cannot have CNAME records.
Question and response
Question
dig mxthunder.net. NS
Answer
mxthunder.net. 4 NS ns.dnsbox.net.
mxthunder.net. 4 NS ns3.dnsbox.net.
mxthunder.net. 4 NS ns2.dnsbox.net.
Authority
The response has no authority records.
Additional
ns.dnsbox.net. 4 A 208.53.48.154
ns2.dnsbox.net. 4 A 45.79.100.232
ns3.dnsbox.net. 4 A 108.60.195.215
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0