Thank you so much for reaching out to us, I am sorry to hear that you are running into this issue, but I would be more than happy to assist.
Due to the issues associated with OS updates, performance related issues, and changes in how DisplayLink works inside of macOS, the UD-3900 is not supported or marked as compatible with macOS based systems. We make sure to point this out on both the Amazon listing and the product page. The UD-3900 is only marked as compatible on Windows based systems, due to a closer integration with the operating system.
The best next step is to help work through providing a full refund for your device. When you have a moment, could you please reach out to us directly at firstname.lastname@example.org, providing your Amazon Order ID Number associated with the purchase, and inputting “Ticket no. 256213” in the subject line so that we can ensure all of your information stays in the same location.
Thank you for your patience,
Product Owner & Technical Support