Hmmm, it seems everything is absolutely perfect, but why doesn't it work? I don't have a CS18AI, so I can't check things, but I have two RM's and know how cascading works. I am using dynamic IP addresses. There is a video fro PreSonus (see link above), which shows how cascading works with CS and two RM's. Have a look if you find anything in the video that is NOT identical to your approach.
I would suggest two things. There is a guy in the forums, who works with RM32AI, RM16AI and CS18A. Ask your question in the StudioLive RM Series forum. Hopefully he has an answer for you. The second suggestion is to open a ticket in your account and ask PreSonus directly.
One last idea: All firmware and software versions are really matching?
UC 1.8 (Mac/PC) 220.127.116.11751
RM Mixer Firmware v9244
CS18 Firmware v9203