How can I obtain the client ID and secret to use the REST API? Are there any fees associated with it?

You can purchase the API access through the control panel. I believe there is a one-time fee of $29.99 to utilize the API with your account.

Please note that I am talking about the full fledged API which allows you to list, add, remove services. The IP update protocol used to update IP address does not require a client ID and secret and can be utilized free of cost.

Client ID and Secret for REST API is now free of cost for any user. You may obtain them from the 'API Credentials' section in the control panel.