@Mesa,

Do you know what type of device it is that is going unavailable? Not all network devices communicate with the network at all times, it is possible that the device has an energy-saving mode or setting that is causing the disconnect.

I would assume that you can access the device again after a refresh, because this may cause your router to ping the devices it knows were recently connected to the network, and this causes the unavailable device to wake and reconnect.

     When you are refreshing, does the IP address of the device change? If so, the router may be trying to look for the device by MAC address, and the refresh forces the new IP to be assigned to the existing MAC address, showing it then as available.

In terms of a likely fix, your best bet would be to set a static IP address for the device, as this will mean the device will always have a path to communicate with the router.

  @Mesa,

There is a system log available, and a debug option available on our Archer Router Web Interfaces, but I'm not sure about our Deco Line.

Are you using a managed switch? If so, it may be a good idea to look at the logs here for any abnormal behavior. 

You never mentioned by the way, does the device actually disconnect from the network/internet? or does the router just display that it is disconnected?