1) (Atlas Mongodb console)First of all click on ALLOW ACCESS FROM ANYWHERE(see in below image) and put some random IP address , don’t click on Add Current IP Address otherwise it will not connect with robomongo .

2) Now open robomongo ,select connection Tab and then select type Direct Connection , and put your primary cluster in Address [you can get your Primary Cluster Address from Project->Clusters->(choose) Primary Cluster-> “There you will find your Primary Cluster Address”] .

3)now click on Authentication Tab , put database name is admin and put your username and password , Auth Mechanism is SCRAM-SHA-1.

4) select self-signed certificate as Authentication Method

5) Now , click on test,we are done !

The standard Mongo URI connection schema has the form:
mongodb://[username:password@]host1[:port1][,...hostN[:portN]]][/[database][?options]]
Security Reasons
- Do not allow access everywhere for security reasons
- Restrict to your IP address
- Connect via roboMongo 3T using a secondary cluster node from MongoDB Atlas