MongoDB free shared cluster is having troubles accepting connections. We are investigating the issue.
EDIT 16/01/2019 09:45 UTC: The problem might be due to old clients drivers being used on the cluster. We have set up a new cluster (version 4.0.3) which should greatly improve things. You can create a new add-on to migrate your database.
To dump your data from your existing, you can use this command: mongodump -u "${MONGODB_ADDON_USER}" -p "${MONGODB_ADDON_PASSWORD}" -h "${MONGODB_ADDON_HOST}" -d "${MONGODB_ADDON_DB}" --archive --gzip
You can then import the data into the new database by using the mongorestore command displayed in the dashboard of your new add-on.
An automatic migration tool for mongodb should be available in the next few days.