Do you have the streamer set in digital preamp mode? If so then it will depend on if CA have given the Roon SDK access to this, not all vendors do. Attenuating via a digital volume is not the best method to attenuate the signal if your going into an amp as you have two layers of attenuation in the chain one competing against another.
If Roon can have access to it, then you would need to ensure it’s in preamp mode, Roons volume setting for the device is set to device volume. If this doesn’t work then Roon is not being allowed access to it.
The digital premap is used mainly for AirPlay and other apps such as Spotify and Tidal connect that attenuate in digital realm, but using this method is lowering SQ compared to the full line out and using your amp to control volume. Your choice obviously.
If Roon can’t see it then not much you can do other than try using Roons DSP volume instead of fixed or device volume, not all devices allow this mode either. But if it can then your best not having the streamer in preamp mode as it will be competing against that as well as attenuation on your preamp.