[ISSUE] Using Apogee Mic 96 with RME [Mac]

Hey guys! So i recently received an Apogee mic 96k which connects as a USB device and I want to use it with my cubase while my main interface is an RME Fireface 400.

I’ve read that i must use the Aggregate device setup to make it work but i can’t seem to make it possible!
No matter what I am doing, I can’t control the volume of the mac from the keyboard for some reason + In cubase after i choose the Aggregate device as the main device and narrowing the Mic 96k i still cannot use it…
(Using a mac pro 2013)

If anyone can guide me maybe of how to do it properly i would highly appreciate it!

Thanks a lot guys !