frame.work/laptop-16-support. Links to software and documentation can be found there.
Numlock on + numpad-enter will cycle numpad brightness.
The keyboards can’t communicate with each other. Each keyboard acts as a standard USB HID keyboard. And USB HID doesn’t provide a way for one keyboard to send arbitrary commands to another.
Full color is only available if you bought a keyboard with RGB in it’s name, or the macropad. All other keyboards have a normal white backlight.