You need to expose the PostgreSQL port directly for you to be able to connect via pgAdmin.
In the app.yml, near the top, you see the 80 and 443 ports open. You can add another line for the port 5432 for PostgreSQL.
That said, this is mostly certainly a very bad idea. The database went from accepting only local connections to being exposed to the whole internet.
If all you need is some occasional reporting, downloading some CSVs from Data Explorer and loading those in your favorite tool may be enough. You can also download Discourse backups (without uploads) and those are just the standard PostgreSQL dump format. With that in hand you can restore it to a local PostgreSQL instance for analysis.