I try to deploy my Mongo database in Mongolabs, everything works fine, and I create a new database. Please see my connectionstring.
public DbHelper()
It's related with MongoDB connection error. Probably you don't have permissions or you didn't specify the allowed IPs in MongoDB. Please check for example in MongoDB Compose if you are able to connect with your MongoDB Atlas. If you won't be able to connect, that means that you have wrong MongoDB connection string.