Under the section of the INF, specify the Modem and the device class GUID. The INF also needs to specify the device setup class as Modem to use mdmcpq.inf.
That INF specifies "Usbser" as a lower filter driver in a device stack. The directive is required for instantiating the service, copying inbox binaries, and registering a device interface GUID that applications require to find the device and talk to it. To load the driver, you need to write an INF that references the modem INF (mdmcpq.inf) by using the Include directive. In Windows 8.1 and earlier versions of the operating system, Usbser.sys is not automatically loaded when a USB-to-serial device is attached to a computer. With the new in-box INF, Usbser.sys is automatically loaded if a USB-to-serial device is detected on the mobile device. Unlike desktop editions, it is not possible to load a driver through an external driver package.
On mobile editions of Windows, such as Windows 10 Mobile, only drivers that are part of the operating system are loaded. Microsoft encourages you to use in-box drivers whenever possible. For information about INF sections, see Overview of INF Files. For examples, look through the INF files included with sample drivers and find devices similar to your device.
If you trying to install a USB device class driver included in Windows, you do not need to download the driver.