1- Download driver from dell Website
http://www.dell.com/support/home/at/de/atbsdt1/Drivers/DriversDetails?driverId=MGMMF
2- Uncompress .exe (with 7zip).
3- enter to folder and edit Configuration.ini and set USBCOMP=8 (change 0 to 8 ).
4- execute .exe (It is in the same folder as the file Configuration.ini).
5- plug card em7455 and open Device administrator. You should see the DELL modem, open a find COM port.
6- Open AT Port (with Putty or other program) and excute this commands:
AT!ENTERCND="A710"
AT!USBPID=9071,9070
AT!USBVID=1199
AT!USBPRODUCT="EM7455"
AT!PRIID="9904802","001.001","Generic-Laptop"
AT!RESET
7- Install Sierra Wireless drivers.
8- Done