Although a thin client responds to a ping command, it does not appear in the UMS Console's list of scanned devices, can not be registered or shows up as offline (red) in the UMS Console's navigation tree.
The packets for scanning the thin clients or checking their online status are getting blocked within the network, e.g. by a firewall or VPN.
Make sure UDP packets on port 30005 are not blocked within your network. Those packets are used for both, scanning for thin clients as well as checking the status of the clients.
See also UMS Communication Ports.