Your linux in Digitalocean probably has included firewall, you need to open port 80.
If you want SSL (https /port 443) you may need to make some modifications to your conf.
You can either enable traefic (i have no experience with it yet) and disable mayan ports. Or you can get an external reverse proxy that does the SSL, like nginx.
I use nginx because i have my own certificate and find it easier to work with nginx than learing how to setup traefik to my use case.
I ended up not using the nginx server. (since cors was already enabled on mayan there were conflicts) the the server hosting the docker image did not have port 80 or 443 open, once i opened those it worked.
I did have to manually edit
vi ./mayan-edms/lib/python3.9/site-packages/corsheaders/middleware.py
and add