To help clarify this, could you please check and let us know which Volume Control Mode is currently selected for your Rose RS520?
You can find this here: Settings → Audio → Rose RS520 → Volume Control Mode
Roon supports different volume control modes (Device Volume, DSP Volume, Fixed Volume), and the ability to control volume from the Roon app depends on which mode is active.
For reference, this article explains the options in detail:
Once you confirm the selected mode, we can advise on the correct setup.