@SBM1950,

If you only perform a soft reset of the device, hold the button for 5 Seconds, compared to the 10 seconds needed for a factory reset, the network settings on the device will be reset and you should then be able to set up the device as you would normally. This will also cause the device to keep its name and configuration options, as it is meant to for reconnecting your device to a network when the connection is lost.

 I believe that the unavailable device in the list should not affect your ability to connect the plug to your network. I believe that this is also true for Factory Resets, as I have been able to readd devices without deleting them.

  @CF4,

Even if your network password and SSID are the same, the devices may not reconnect if there are other settings that make the network look different than the previous. 

The solution is to use a soft(5 Sec) reset and reconfigure the network settings for the device, otherwise you can delete the devices in the Kasa App by tapping and holding on a device name, then you can perform a factory reset to reconfigure the plugs