Get a stereo preamp with a remote.

What model is everything in your setup that your are trying to control the volume remotely.

Why would it need to be controlled from a smartphone?

If it must be from a smartphone:

• Acrylic S10 direct or from Amazon seems to be able to control the volume of the aux input to output.
• Acyrlic S50 Pro adds RCA in and out instead of 3.5mm.