Apex domains cannot have CNAME records.
Question and response
Question
dig mongo.ondigitalocean.com. NS
Answer
mongo.ondigitalocean.com. 5 NS ns1.digitalocean.com.
mongo.ondigitalocean.com. 5 NS ns2.digitalocean.com.
mongo.ondigitalocean.com. 5 NS ns3.digitalocean.com.
Authority
The response has no authority records.
Additional
ns1.digitalocean.com. 4 A 172.64.52.210
ns2.digitalocean.com. 4 A 172.64.53.21
ns3.digitalocean.com. 4 A 172.64.49.209
ns1.digitalocean.com. 4 AAAA 2606:4700:52:0:0:0:ac40:34d2
ns2.digitalocean.com. 4 AAAA 2606:4700:5a:0:0:0:ac40:3515
ns3.digitalocean.com. 4 AAAA 2606:4700:52:0:0:0:ac40:31d1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0