Before connecting to an Atlas cluster, ensure your application can reach your MongoDB Atlas environment by allowing your applications to make outbound connections to ports 27015 to 27017 to TCP and UDP traffic on Atlas hosts.
To connect to an Atlas cluster:
Configure an IP access list entry on the Atlas project’s IP access list for Network Access
Create a Database User with access to your database(s) for Database Access
Copy your cluster's connection string by hitting CONNECT on your cluster and following the instructions for your chosen connection method
For instructions on connecting to your Atlas Cluster see:
For troubleshooting connection issues:
Data Explorer for managing Atlas data
See also the article Connections in MongoDB Atlas Clusters explaining Atlas connections and where to see them.