Thank you for contacting Plugable support. We are sorry to hear you are experiencing this issue and would be happy to help assist.
Just so I understand your setup correctly you have a PC connected to your UD-3900 with a USB cable. Would you happen to be using a USB 3.0 Type-B to USB Type-C cable? And then you have your two wimaxit 15,6 inch monitors connected to the UD-3900 with HDMI cables? Are you using the DVI to HDMI adapter for one of your wimaxit 15,6 inch monitors perhaps?
Would my understanding be correct here? If not please feel free to correct me as I want to be sure I understand your setup.
In addition when you have a moment, could you please collect a set of diagnostic logs generated by our PlugDebug utility? To collect these logs, please have the device plugged into your computer, and please follow this link for instructions: https://plugable.com/pages/plugdebug. The tool will collect system logs into a ZIP file and save them to your Desktop so they can be attached to your email reply. These log files combined with the problem description you’ve provided may help us understand what’s happenings we can then provide the next best steps.
Please do not post your log files here. If you could please contact Plugable support directly when sending your log files that would be appreciated. To do this, please use your preferred search engine to search for “plugable support email”. This should get you to the web page with our direct email address. In the subject of your direct email, please reference case 345581 which will allow us to match things up and then provide the next steps.