It works with non-Apple displays (I use a Dell 4K monitor with my Mac Mini), but the OS has to be able to detect that the pixel density is suitable. It’s possible that macOS can’t discern enough about your monitor.
If you option-click on the resolution options you may be able to manually override it.
Careful with that setting. Any non-integer scaling like 1.5x will force macOS to render everything in 5K (I think?) and scale it down to the target resolution - this is way more taxing on GPU, battery life etc. than 2x scaling which simply display all content twice as large (everything is delivered with 2x assets after all).
If you option-click on the resolution options you may be able to manually override it.