By default, if an Atom Worker is enabled on an Atom Cloud, the Default port range is 20000-30000. It will start at the minimum and increment by one until a valid port is found.
You may also want to consider adding a specific port range for your Atom Workers that you know is opened up via your firewall.
The port range can be configured by setting the property Atom Worker Local Port Range property in Properties panel.
In either case above, the port range may need to be opened to appropriate traffic by your network security administrator.