Robomongo: Cannot connect to replica set. Set's primary is unreachable

First of all, check if you are using latest Robo3T. MongoDB Atlas uses mongodb+srv:// protocol, that was not supported some time before.

After that please go by following check-list:

• Use import From SRV by copy-pasting URL from MongoDB Atlas;
• MongoDB Atlas is using Replica Sets even for free Sandbox (Atlas M0) accounts. So during creation ensure that you used that type;
• Fill or check all cluster members;
• Set Name should be set.

important differences on Authentication tab:

• Database should be admin not like database name (used on Heroku),
• Auth Mechanism should be SCRAM-SHA-1
• Check Manually specify visible databases and type there admin,<you database name from Heroku>

on TLS tab:

• Check Use TLS protocol
• Authentication method - Self-signed Certificate

Now it should work.

P.S. Previously, I also had problems with having database same like on Heroku with same db-name and user-name, also I thought that Auth Mechanism should be MONGODB-CR. But that didn't work.

P.P.S. If you don't see Manually specify visible databases - then please check if you have the latest version (1.4.+) of Robo3T (previously known as Robomongo)