Thanks for the reply, @girish.
host -t NS
lead me to find that the NS records were not added in DigitalOcean.
Adding them in fixed the problem.
I was alerted that one of my domains was no longer reachable yesterday and when I went to go visit it found that the Let's Encrypt SSL was broken.
I tried to refresh the cert in Cloudron but got the error "queryNs ENODATA <domain>"
Nothing has changed for this domain, including DNS. How do I go about fixing this?