The "Unable to reach device" or "Device Unavailable" message occurs when the device stops talking to the XOGO backend. This means that your network was interrupted. To resolve the issue a few options exist.
1) Digital Signage loves a wired network connection. Can you give the player a hard-wired connection? If so, this will resolve the issue permanently.
2) If we are Wi-Fi, is it possible to move the player closer to your Wi-Fi router/repeater? This may resolve the issue although WiFi is never a perfect solution for 24/7 services like digital signage.
3) For a quick fix, you can restart the player. However, this is typically a patch for an underlying issue. (No Wired connection or Poor WiFi)