Our CCS installation (12.5.2) has issues fetching the secure key from agents.
This has been found to be caused by a firewall, such as a local firewall on a Unix asset.
For example, iptables or nftables running on a Linux server.
Create a rule on the firewall(s) to allow the communication with the agent to go through.
Ports used by the Agents
5601 - Windows Agents
5600 - Unix Agents
5599 - Used for APU/ACU jobs to update the Agents.
Here is an article to assist with troubleshooting connectivity.
https://knowledge.broadcom.com/external/article/195968/using-telnet-to-troubleshoot-connectivit.html
For Agent based troubleshooting take note of what will get logged in the Agent log file, when an incoming connection is made outside of CCS.