Fibaro 2 Dimmer associated with a permantely wired Fibaro RGBW

I think you can do what you are trying to achieve using the Fibaro Dimmer Scenes app.

You would need to look in the recently screen for your Dimmer 2 and see which scene number is activated when you turn your switch on and off. Then in the dimmer scenes app set those scenes to switch on or off your RGBW strip.

Check out this thread for more information.

I have a Dimmer 2 and have added a 2 gang switch, the second switch is connected to S2 on the dimmer and I use the fibaro dimmer scene app to toggle the two lamps in that room.

My RGBW lights in my kitchen are controlled by routines or by voice using an Amazon echo. I have no switch for them.