Control AppleTV 4K volume on monitor with speakers
There's no easier way from the AppleTV or similar. In modern HDMI based systems the volume control always happens at the destination (for example the TV). When you send the audio digitally, the source (Apple TV) is not amplifying an analog signal and thus has no impact on the actual volume except for any external control signals it can.

Your computer monitor is probably not designed for use as a TV, and does not support receiving volume control signals anyway other than the on-screen menu. According to its manual, there doesn't seem to be any remote controls available or similar.