If it still does not appear in device manager then you should contact the original manufacturer, not DisplayLink, to discuss a hardware replacement of the product.
Unplug all cables from the device, then reconnect power (if needed) then the USB cable.
Check the USB port with another device to make sure other devices are recognised.
Check that the USB cable works with other devices.
If using a USB bus powered device, make sure that the device is connected to an externally powered USB hub (the hub has a USB cable and a power supply connected to it). This will ensure that the device an receive enough power from the USB bus.
Try the adapter on another computer.