Did you read this?
http://answers.presonus.com/9890/cs18ai-cannot-detect-the-rm32ai-direct-ethernet-connection
"After many, many hours trial and error and countless hours researching all the info I can find online, I have finally gotten my CS18AI to work with my RM32AI with ONLY the ethernet cable installed. No need for the Wi-Fi dongles and a router. I can finally use the RM32AI as a stagebox to my CS18AI. I verified the connection by removing the wi-fi dongles on the RM and the CS, switch off the sets and turning them back on again without the dongles installed. Only the CAT6 cable was connecting the sets. It all works fine! I siwtch off and on the sets many times and it still works. I am finally able to use the RM as a stage box to the CS without having to lug a router along to the set. I used 80m of CAT6 cable between the RM and the CS for an event and it worked fine. The trick is to set both the CS and the RM mixer to a static network address with a common gateway."
