The Airport isn't acting as a router is it (I don't know a lot about it the Apple stuff!)? Have you told the Roamio Plus to act as a MOCA gateway instead of just ethernet?
I believe you need to select the "use this DVR to create a MoCA network" if you haven't already, were are you seeing the port mapping question prompt? Is that on the TiVo or the Airport extreme?
Unless the Airport Extreme isn't acting as a router then there shouldn't be any need for any ports to be remapped as that's typically something you need to to do with your internet router so that apps like multi-player games or communication apps like Skype might to talk to something outside your network.
When I setup my Mini the first time, I used ethernet on a good network connection then moved it to MoCA and to the room it was to be in, after I knew it all worked correctly. That way it was easier to figure out what was wrong in case hooking it up to MoCA right away didn't work.