Thanks for posting. I’d be happy to help!
In looking at the manual for your monitor --> http://downloads.dell.com/Manuals/all… the specifications state that in order to reach the maximum resolution of 2560 x 1080 over a DVI connection a dual-link DVI device is required. You don’t mention the exact Plugable product you have, but all of our USB video adapters with DVI outputs are single-link devices, and the maximum resolution our newest DVI adapters support is 2048 x 1152. Again looking at your manual, the listed supported display modes jump from 1920 x 1080 to 2560 x 1080 with nothing in between. As the monitor cannot honor the request from our adapter to run at 2048 x 1152, it drops down to the common denominator of 1920 x 1080.
All is not lost however…you could purchase one of our UGA-4KDP adapters --> http://plugable.com/products/uga-4kdp and connect that to your monitor using the available DisplayPort input and that will drive the display at its full resolution. As we offer a 30-day no-hassle return policy on all of our products, you can return the current DVI adapter you have to Amazon for a full refund using the automated system at amazon.com/returns.
Let me know if you have any additional questions…