What i did to reconnect K57 with the dongle:
Plug in Keyboard over USB.
Plug in the dongle.
Start iCue Software (version 4)
Go to Dongle / Connections
Press initialize.
Now they are connected again.
Remove Keyboard from USB.
P.S. Be sure that wireless connection is turned on (Press FN + F5)
A flashing white light indicates not connected with dongle.
A static white light indicates connected with dongle