If you do not see your device listed in the Connected Devices or Disconnected Devices sections, that means it hasn't connected to the router yet. You will need to connect to the router's WiFi network or via Ethernet and then refresh the Per Device VPN page. You will see your device listed under Connected Devices.
If a device disconnects from the router, it will be added to the Disconnected Devices section. Even though the device isn't on the network, you can still edit it's properties in the router firmware.