To be honest, I think the problem is my lack of skills. I've tried several times to get ODK Central set up. The instructions are really great for the ODK side of things and I suspect I'm doing something wrong with my domain name. I first tried FreeDNS but then couldn't see how to set up the domain name for my droplet. I switched to Google Domains as the DigitalOcean's guide for that one seemed straightforward. I looked through the forum and found the conversation about droplet size, so deleted my droplet and started again with 3G. Still no luck.
Is there any way I can check if my problem is to do with my domain set up or ODK Central?