The agent tries to contact the server on netbios name, then FQDN and at last at ip address stored in the agent config files. If the DNS settings of the client appears to be incoorect, the agent is getting an IP address when asking the DNS with the FQDN.
If it cannot contact the DNS server at all, it is trying the IP stored in the config files.
In your case, the DNS server was not been able to point to the correct IP. The sensors can therefore not connect to the server and checking if the agent has been installed..
Remember, the sensor is not deployed to rough machines, it should be deployed on managed machines. It's task is to detect the rough machines and then, depending on the settings, installing the ePo agent on the rough machines.