Most laptop miniPCIe slots do NOT support USB, but it is very difficult to tell the difference.
The mini PCIe form factor only defines the physical dimensions of the card and slot, not the electrical specifications, therefore the slot can (and is) used for both USB, PCIe, and SATA connectivity and not all slots implement all electrical standards.
This is a source of much confusion which was why the NGFF (m.2) standard explicitly addressed this by adding keying tabs to indicate which slots and cards had USB/SATA/PCIe.
In your particular case the chances are 100% that it will not work unless your laptop already has a SIM card slot. Dell boards often have multiple Mini PCIe slots, but you will need one marked "WWAN" to have any chance of it working. As far as I'm aware, some Inspirons had this option but most do not.
Remember that in addition to the USB interface, the board must also have a SIM card slot and a way for the 3G card to communicate with it. This is hard-wired into the MiniPCIe slot, so a laptop must be designed with WWAN (3G) in mind from the outset.