Sometimes even after correctly configuring DNS for your new Discourse site hosted with us:
You still cannot access the site and only reach an error page:
If this persists for a while (nameservers will cache negative responses for a while), we will ask you to check to see if you can access from a mobile device (with Wi-Fi off).
If this allows you to access the site then the cause is likely Split Horizon / Split View DNS:
(image from https://ns1.com/resources/split-horizon-or-multiview-dns)
In essence: there are two different versions of DNS for example.com. The change adding the record:
community.example.com. IN CNAME example.hosted-by-discourse.com.
was made only to the external version and not the internal version.
To correct this, the record above needs to be added to the internal version to match the external version.
(n.b. there is nothing specific to our hosting about this, but it comes up frequently)