21810.example.com has no CNAME records.
Question and response
Question
dig 21810.example.com. NS
Answer
The response has no answer records.
Authority
example.com. 1785 SOA elliott.ns.cloudflare.com. dns.cloudflare.com. 2397268091 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0