Well, maybe the card is just deactivated by default to save battery. You can enable it easily by sending:
to it (AT+CFUN? returns the current state, AT+CFUN=1 enables the device) with a terminal program of your choice (i always use "screen /dev/ttyUSB0" for such tasks). At least the card remembers this setting, so you don't have to repeat it every time. However, a setting somewhere in UMTSmon to automatically deactivate the card when the program exits would be cool to save some more battery power... I will have to look into that.