First, POE will not power your mixer. Adding an AVB switch will allow you to use the inputs on the CS18AI and still have a wired connection for your wireless router, and recording via AVB.
You have three choices that I know of:
1: You can hook the CS18AI to your router via an Ethernet cable, or:
2: Set up your RM32AI and CS18AI wirelessly with the wireless dongles, or:
3: Set up your RM32AI wirelessly and run an Ethernet cable from the CS18AI. This option will still allow you to use the inputs on the CS18AI for say; talk back, stereo break music playback, and a wireless mic...
Here are some PreSonus video links:
And this setup guide:
Hope that helps!