if the Mac is in another room you have to go to that other room, select the devices, then you will be able use Audirvana. If the devices are not selected on the Mac then Audirvana cannot “see” them. So although, yes, you can use the remote app for Audirvana on a mobile phone to select the output, that is only the case if the outputs are available to Audirvana on the Mac and for the Airplay 2 device to be available it has to be selected as the output on the Mac.
Would you like to remotely control your Mac from your mobile device (assuming it’s an iPhone or iPad)? If yes, go to the iOS App Store and type “VNC”. They come in all shapes and colors 🙂 . The iOS app named “Screens” v.4 from Edovia has been well-reviewed in the Mac press, though I don’t own it myself, and have no affiliation with any vendor.