Control Harmony volume with AppleTV remote

We have a Harmony hub to control an Onkyo AV receiver, a projector and a 4K AppleTV. All the gear is located in a cabinet outside of the theater room. Often, we like to use the AppleTV remote to select content. But, we must grab the phone to control the volume thru the Harmony app (or ask Alexa to do it, but that’s a whole nuther ball of wax).

Q: does anyone know of a way to control Harmony, specifically volume, using the AppleTV remote? N.b. IR doesn’t work because of the equipment location, and the receiver is too old for HDMI-CEC.