I use DSR to scale up to 1400 for the HMD, but after I got 1809 (I got the early update before the cancelled it for everyone), it refused to do any DSR (with HMD or not). I think this was some error in the update that wasn’t allowing Nvidia’s driver to use scaled resolutions (or enable higher resolutions than my monitor can handle (but the HMD can)).
I don’t know if anyone else will have this issue, but in case you do, here is the solution: make sure you have the latest Nvidia driver installed (seems obvious, right? but:), then go back in to the Nvidia control panel and re-enable DSR (it is disabled regardless of whatever your previous settings were).