Unmanaged means client system is not communicating to ePO server.
Suggest you to try the below steps:
1. try to ping ePO server from client system
2. try telnet on port 443 and 80
3. Share the agent log from client system
check if you have duplicate system in ePO system tree with the same system name
Usually, installing the latest agent fixes it. If you have admin rights on the system you push it.