Thanks. Yes it works.
I use the Naim App and (A) disable the switch “automatic”. Then I (B) switched the Volume Control to variable. (already done before)
I disabled the device in Ronn.
Then I stopped the Roon Service and pluged off the Naim Streamer.
After restarting the Roon Service and plugging in the Naim Streamer, I “activate” the Streamer as ROON RAAT device with the Roon App.
Now it was possible to set the Volume directly in the Roon App.
Thanks to Andy for the hint.