Switching database URL on Heroku between two AWS RDS instances has no effect

I am running a django app which connects to heroku. My database is configured to use AWS RDS.

Now I needed to change db instances. So I created a new instance on AWS and changed the config vars in Heroku.

The weird thing is that it has no effect at all. Heroku still connects to my old AWS database.

Can it be that Heroku doesn't overwrite the DATABASE_URL or sth similar?

I did:

heroku config:set DATABASE_URL="postgres://postgresusername@awsendpoint/databasename?sslrootcert=rds-combined-ca-bundle.pem" -a appname

heroku config:set POSTGRES_DB=nameofdatabae
heroku config:set POSTGRES_DB=nameofdb.somenumbers.eu-central-1.rds.amazonaws.com

When I do heroku config I can see that my config vars are updated. I also restarted the app and deployed anew. But the changes still don't take effect.

Is there anything I am missing here? Or how can I debug this?

Help is greatly appreciated. Thanks in advance