I just got this working, and well I might add. Four quick questions.
Is there an AVB switch between the CS18AI and the RM (16/32) unit? Using headphones, mic 1\2 and lines 3\4 require a AVB switch. I am using a MOTU switch.
Press the UCnet button - is the CS18AI registered to the RM (16/32) unit? I assume you already have done this, but it doesn't hurt to ask.
Have you moved the digital line return fader up (press the RETURNS button - that fader should be marked 'digital' on one of the scribble strips)? The inputs marked 3 and 4 are sent via a digital return to the RM (16/32) through the AVB network.
Is the iPhone output all the way up? I am using a 6S and for this device, I raise the volume to max. You may want to be very careful - and have the dgital return fader off when you start a song, then raise it slowly - my results may be very different than yours.
Have you updated the firmware on the CS18AI, RM (16/32) and AVB switch so that each is current?