How to use DSR on a non-native resolution.

Hi! I own a 21:9 2560x1080 monitor. When I choose DSR 4x I get this resolution - 5120x2160 - which is 21:9 4k. I want to use DSR on 1920x1080(a resolution that I have available for use) so I can get 3840x2160. The reason is obvious. Most games (new or old) don’t support 21:9 and I don’t have the money for a better monitor right now. I know that it’s not the end of the world or anything, but it’s still a shame that Nvidia doesn’t allow me to do that. DSR is amazing and I would love to use it in every game. If a developer of the DSR technology or another smart person sees this thread can you please tell me how to get DSR working on a non-native resolution that 100% supports DSR?