Hi,
Did you try with the latest drivers? it should work for UART. Normally for UART, you can select COM1 which is stable even after restart.
You can try the below drivers… Hope will work 
http://source.sierrawireless.com/resources/airprime/tools/usb-driver-installer/
http://source.sierrawireless.com/resources/airprime/tools/airprime—usb-driver-for-windows-7-and–8,-d-,1_sl808xt-series/
-Alex