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Heya,
Just came across this answer and decided to write some general guidelines for anyone who comes across this in the future despite the old question.
It seems like your application is not reachable despite Dokku running and port mappings set correctly. There may be a number of reasons causing this issue. Ensuring that your application is running correctly would be the first step. You may use the command dokku ps:report to check the state of your applications.
If your application is running correctly then the issue could possibly reside with the networking or firewall settings of your DigitalOcean Droplet. Ensure that the firewall on your Droplet allows incoming connections to the ports that your application is running on. You’ve already mentioned ufw firewall but you should also check DigitalOcean’s Cloud Firewalls if you have them enabled.
Since Dokku uses Nginx for its network interface, another possibility could be an issue with the Nginx configuration. You may wish to check the Nginx error logs for any problems.
Kindly refer to these DigitalOcean docs for more detailed help:
Hope that this helps!