DigitalOcean Port Closed after Installation

[JammyDodger]

This has cropped up previously, eg:

Digitalocean 1-click Discourse install

I am getting an error while trying to complete the Digitalocean Discourse 1-click install. Checking your domain name . . . WARNING: Port 443 of computer does not appear to be accessible using hostname: discourse.openfarming.net. WARNING: Connection to http://discourse.openfarming.net (port 80) also fails. The droplet’s IP address is 206.189.7.42. The nameserver for openfarming.net is TMDhosting, and using the cpanel there I have the following zone records for the domain: NAME …

In that case they had a much easier time moving away from the one-click installer, and following the standard install guide instead.