A timeout occured after 30000ms selecting a server using CompositeServerSelector

Add "?connect=replicaSet" to the end of your connection string if connecting to MongoLab.

new MongoClient("mongodb://username:[email protected]:11111/db-name?connect=replicaSet")

This JIRA ticket has some details: https://jira.mongodb.org/browse/CSHARP-1160

Basically the default is to connect to a replica set member. But MongoLab's Single-Node settings are actually a single node replica set and this causes us to not trust it. Appending ?connect=replicaSet to your connection string will force the driver to move into replica set mode and all will work.

Found that info here.

I am replacing the connection string method in like below.

new MongoClient("mongodb://username:[email protected]:11111/db-name")

Now it's solved.

Please see the answer from Paul Lemke.

Make sure your current ip address is white-listed in mongo db server. If you change your internet provider new IP needs to be white-listed.