Huewei E1750 modem not recognized by the KDE applet. "Mobile Broadband" checkbox does not appear when clicking on applet. The indicator on the modem just blinks forever, no window with PIN request appears. Works perfectly when running gnomes' nm-applet, able to enter the PIN and the modem then also appears in KDE applet. Used to work in KDE 4.10. Broken after upgrading to FC19 with KDE 4.11 and KDE 4.11.1. Reproducible: Always Name : NetworkManager Version : 0.9.8.2 Release : 8.git20130709.fc19
What configuration do you have in "Manage Connections" -> Other -> Ask for GSM Pin? Try changing that to "On Modem Detection". By the way, I have just checked and my Sony Ericsson MD300 modem works perfectly here, so if the option above does not help then it is probably a bug somewhere else.
I have got "On modem detection" set in "Ask for GSM pin" setting.