In order to solve the issue, you need to set your username and password to guest. Also, I suggest you reinstall the USB Modem drivers. For this purpose, press Windows + Pause/Break key combination and open Device Manager. Expand the Universal Serial Bus controllers and Network Adapters lists, right-click on the modem driver and select Uninstall. After that, reboot your computer, connect the USB modem, press Scan for hardware changes icon from Device Manager and reinstall the drivers.