I posted here some 4 months ago about the device unplugged problem I was having with the ZTE MF112 mobile broadband dongle and the TP-Link mr3220 Router. The solution was to use a different dongle namely the Huawei K3565. This worked flawlessly for 4 months until I started getting the device unplugged message with the K3565 out of the blue.
So I ended up hard resetting the router by using a pin to reset it on the back side of the router for a few seconds and since then I have had no further problems. Incidently I tried the MF112 after a hard reset and it still didn't work properly.