Meteor is an excellent platform for reactive web applications. It supports Mongo DB as its primary Data Store layer. It comes with local database for development purpose. But for a real production deployment as in most of the cases, a remote database needs to be associated with the application. Meteor reads MONGO_URL parameter from PATH to determine if it needs to start a local database.
Important factor here is to point MONGO_URL to correct Mongo DB instance.
If your MONGO DB port is
27017, you can skip port in below URLs.
Go Computer->Advanced-> Environment Variables
heroku config:set MONGO_URL=mongodb://yourdbaddress:port/yourDB
You should get success message something like this
Setting MONGO_URL and restarting ⬢ <appname>... done, v10 MONGO_URL: mongodb://yourdbaddress:port/yourDB
Hopefully it is helpful.