If an edynamo reader is disconnected its usually one of two things.
It either needs to be reset or there's an issue with the lava adapter.
To reset the reader, hold the little button on the bottom of the reader for about 10 seconds. The green light on the side should flash red to indicate the reset.
In the Tray app click the red dot and then click Reset Reader.
Ideally a prompt should come up with a checkbox to use the USB device by default. Make sure to check the box and hit ok. Usually the prompt comes up twice so check it both times. After the prompts the card reader should be connected and ready to use
The tablet may need to be restarted to get the prompts.
Currently (April 2018), the more common issue is the lava adapter not working correctly and should be replaced. To get around this the reader should be connected via bluetooth.