No volume control on Dell U2718Q Monitor with Macbook pro

I have a 2014 macbook pro hooked up to dell ultra sharp 2718q monitor.

i have a oontz angle small portable speaker unit that i have hooked up to the monitor via the monitor's audio output jack

the speaker works but the volume is very high and i can't turn it up or down using the mac system controls (either the keyboard buttons or the system controls on the top right of the screen)

buttons on the actual speaker for volume don't work either.

the volume controls do work fine when plugged directly into the macbook headphone jack

anyone else have this issue or any potential solutions?