RoonARC USB Driver can not change volume when using DACs do not have phsic volume controller

I tried Focal Bathys DAC mode with RoonARC. It works but I can not change the volume because Bathys does not get a physical volume controller. When I press the volume +/- buttons on the headphone they only change the Android system volume. The RoonARC playback volume is stuck.
Then I connect a set of DAC & Amplifier which has physical volume dial and I can tune the playback volume.
If RoonARC takes control of the USB DAC, it should also control the volume itself.