Apple tv volume control and Asus computer screen

Is your remote control an Siri Remote
Use your Siri Remote or Apple TV Remote with Apple TV (4th generation)
https://support.apple.com/en-au/HT205305
And
Apple guide states>Go to Settings > Remotes and Devices.
Under Home Theater Control, enable Turn On Your TV with Your Remote.
Go to Volume Control.
If the options are available, you can select Auto or TV via IR.
 If not, click Learn New Device to map your Siri Remote to your television.
How to control your TV with your Siri Remote
http://www.imore.com/how-use-your-apple-tv-remote-control-your-tv

Thanks for the effort, but I'm not using a tv. I'm using a computer screen.

Oh I see sorry, the Asus vp278h is a monitor no TV? or computer and no wireless internet access? connected .
  so you have connected the apple console to the Asus vp278h monitor.
https://www.cnet.com/products/asus-vp278h-p-led-monitor-27/specs/

Maybe you have the wrong cables connected to this Monitor.
The hdmi usually have sound included.
Otherwise use the vga connection.
Have you tested the touch pad on top of the remote control?
Could be a setting.
Have you checked the manual?
I cant show you the settings on the screen, however, if ever you get stuck and need to see" check out youtube.
Please see this video, shows the correct cables you should have and what to connect to what.
Because the speakers are inbuilt possible a setting can be changed.
How to connect Apple TV to computer monitor
https://www.youtube.com/watch?v=T2EyTBzLvFU
New Apple TV 4 HDMI CEC Control Volume and Power
https://www.youtube.com/watch?v=c85JB2e7qac
How to Set Up an Apple TV
https://www.youtube.com/watch?v=Xj_pNWehNjA

Normally Apple TV are only compatible with Apple Products.
Let me know how it goes and good luck,
Regards Merete

No dice unfortunately...

Do you see the Apple TV Main menu?
Any details help.

Only newer model TVs with HDMI-CEC will be able to be turned on by the Apple TV remote, but certain TV models should theoretically be able have the volume control adjusted by the Apple TV 4 Siri Remote using the "HDMI-CEC"
“HDMI-CEC”, short for HDMI Consumer Electronics Control, is an HDMI feature many TVs and peripherals have.
 This feature makes your devices work better together, but is often disabled by default.
 If your TV has any of these features, it supports HDMI-CEC.
How to Enable HDMI-CEC on Your TV,
HDMI-CEC Trade Names
You often won’t see “HDMI-CEC” printed on a specifications list.
 Instead, you’ll see a branded “trade name.”
These names all refer to HDMI-CEC, so they really exist only to confuse customers.
 If your TV has any of these features, it supports HDMI-CEC.
 You’ll need to know the name your TV’s manufacturer uses so you can hunt down and enable the disguised HDMI-CEC option on your TV.
AOC: E-link
Hitachi: HDMI-CEC (Thank you, Hitachi!)
LG: SimpLink
Mitsubishi: NetCommand for HDMI
Onkyo: RIHD (Remote Interactive over HDMI)
Panasonic: HDAVI Control, EZ-Sync, or VIERA Link
Philips: EasyLink
Pioneer: Kuro Link
Runco International: RuncoLink
Samsung: Anynet+
Sharp: Aquos Link
Sony: BRAVIA Sync
Toshiba: CE-Link or Regza Link
Vizio: CEC

refer to this to see how to anyway
Control your TV or receiver with your Siri Remote or Apple TV Remote
On your Siri Remote or Apple TV Remote, press  Volume up  or  Volume down on the Siri Remote  to turn the volume up or down.
 If the volume doesn’t respond, follow steps to manually program your Siri Remote or Apple TV Remote for volume control.
https://support.apple.com/en-au/HT205225

It turned out it was not possible to control volume levels in this way