My best guess is that you did a bunch of rebuilds with something misconfigured and now let’s encrypt won’t issue a certificate because of rate limiting.
If that’s the case you can wait a week or try using www as the subdomain, which is really a good idea these days anyway.
You can look at the logs in /var/discourse/shared/log/var-log/nginx/access.log or perhaps
docker logs app
I expect you’ll see issues with the certificate not existent or invalid.