Interesting that Dell didn’t add the ‘4K’ downsampling setting with the S2522HG, seems they often miss this out on their monitors. But that shouldn’t matter as a PC user – as suggested in my previous reply, you can use DSR (‘Manage 3D Settings’ – ‘DSR – Factors’ in Nvidia Control Panel) to create a ‘4K’ resolution. That will get the GPU to output at the ‘4K’ resolution and should work for Nvidia’s GameStream service as you describe. In theory at least, I can only assume it’s detected in the correct way to allow that sort of thing. This mode works in a similar way to a ‘4K’ downsampling mode that some monitors include. It’s handled by the GPU rather than the monitor, but it provides similar output from what I’ve seen. Sometimes you can create a custom resolution instead, but that doesn’t always work.
Edited this reply for accuracy as custom ‘4K’ UHD resolutions don’t always work.