I use ePO 5.1. DLP policy is set to block all external USB devices except a few exceptions whose VID and PID are added. Huawei and ZTE dongles' VID and PID have been added to the exception list. But the devices are still being blocked, and if allowed, not able to access the application in them.
Goto DLP incident manager and open the specific event and check which rule is blocking the device.
On the same page, click on actions and click "export device parameters".
Open the DLP policy console page, goto device definiton and import the exported device parameter. If it is a PnP rule, import it as a plung and play device definition.
Exclude the specific device definition in that rule, it should work.
Generally, you will get two different events when you insert dongles on the client machine. So, export both the parameters and import it in DLP policy page.