How to control the volume of an Android device from an iPhone? How to control the volume of an Android device from an iPhone?
You could use my free open-source and no-ad app, Web Remote Volume Control, I have made it myself to address this exact purpose, see my answer on this thread.
Full disclosure: I am the author of this free, ad-free, and open-source app.
You could instead use Teamviewer plus a slider widget (Teamviewer Remote Control on the iPhone, Teamviewer Quick Support on the Android device, and also a sound volume slider widget on the Android device), but this may not be free.
You could maybe also use Airdroid, but I think it is not free either.