Thank you so much for the reply Pluga_Bob.
As it turns out, I managed to get it working but not by this method.
Before you had a chance a reply I found another thread in these discussion boards where someone had something similar going on. That made me think I had perhaps installed it incorrectly.
Sure enough, when I did exactly the steps in exactly the order outlined by Mitchell, the printer was recognized, and continues to be even after hibernation.
https://support.plugable.com/t/installation-issues/18985/2
Not sure why the UD-3900 didn’t come with installation instructions–I bought it new–but it all worked out in the end.
Thanks again for your detailed reply. I very much appreciate your time & effort!