The list you have for outbound access is for the Atlas Control Pane
If your network allows outbound HTTP requests only to specific IP addresses, you must allow access to the following IP addresses so that your API requests can reach the Atlas control plane:
That is the API you can use to manage your cluster(s) not your Atlas cluster.
You will have to lookup the IP’s of your atlas nodes yourself.
An example of how to lookup your cluster nodes you can see in this response: