Apple

If you are interested in a secondary remote, we offer an infrared or Bluetooth Button Remote for Apple TV. It will control Apple TV by IR or Bluetooth and control the volume, mute and power of your TV, receiver, or soundbar.

https://function101.com/pages/button-remote-for-apple-tv-home

We have a Cyber Week offer for 30% off with code REMOTE30

if you're using IR to control the volume, then the phone remote won't work

If you're using HDMI-CEC to control the volume, then the phone will work.

CEC is consumer electronics control. it allows you to send commands over the HDMI cable. When enabled, it will also let your TV remote send basic navigation commands to the aTV.

ARC is audio return channel, Has nothing to do with CEC, but they're often available together. Manufacturers call it different things (simplink and anynet are 2 examples) you'll have to google your TV's model and CEC to figure out what they call it. a lot of times it's disabled by default.

Check the remotes section of the aTV's menu, at the bottom in the volume control section, you'll want it to say "by HDMI"

you can try unplugging and replugging both ends of the HDMI cable, which will cause your devices to handshake again, maybe something got corrupted and it forget you can do CEC. If you're using a lower quality HDMI cable, I'd think about replacing it. Doesn't have to be super expensive. Look for HDMI 2.1, monoprice makes good quality cables, you can order through them, or from amazon, should be under $20

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There are several cases for the Siri Remote, the one I have is big enough for an airtag behind the remote. Makes it big enough to not slip between the cushions as easliy, and if it's really lost, I can make it ping, or use precision finding to figure out where I carried it off to.