Apex domains cannot have CNAME records.
Question and response
Question
dig mangohost.net. NS
Answer
mangohost.net. 1 NS ns2.topdns.me.
mangohost.net. 1 NS ns3.topdns.me.
mangohost.net. 1 NS ns1.topdns.me.
Authority
The response has no authority records.
Additional
ns1.topdns.me. 1 A 192.0.2.2
ns2.topdns.me. 1 A 88.208.36.16
ns3.topdns.me. 1 A 46.229.175.55
ns2.topdns.me. 1 AAAA 2a02:b49:4:3:1:0:0:1
ns3.topdns.me. 1 AAAA 2a02:b48:6:25:1:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0