I was on tech support numerous times with TiVo before finally getting my Roamio and TiVo mini to work together. The basic problem is the documentation for how to hook up these devices is incorrect, incomplete, and misleading. Once I figured it out and got it working, I love these devices. They let you set up a central media center in your home and share it to several rooms in your house, with just the small footprint (and diskless/noiseless) Mini in your other rooms. I particularly like no longer hearing the disk noises in my bedroom.
In the hopes my efforts to figure this out can save someone else the same hassle, here are my notes. TiVo - please consider fixing your documentation per my notes below! It had some very misleading flaws, and now that I understand what I'm doing the setup actually is pretty straightforward.
First, ignore the instructions that come with the TiVo that you can use Ethernet to connect the Roamio and Mini. I tried this, and the video quality at the Mini was horrible - it would freeze, skip, and barely play at all. You need to use MoCa, which provides much higher bandwidth connectivity (up to 800Mbps ostensibly, vs 100 for typical home Ethernets), as needed for transmitting hi def video.
Where the coax cable enters the premises, attach a point-of-entry filter (which you can buy from TiVo). This is needed to isolate your local coax loop from other units in an apartment complex (may not matter as much for a house), for 2 reasons: (a) avoid signal leakage, which degrades ability to transmit high def video; (b) avoid other apartments' being able to get on your Internet connection.
Attach the point-of-entry filter to a 2Ghz splitter. Don't use the 1Ghz splitters you get from the cable provider - they won't pass the high bandwidth signal used by MoCA.
Connect the output of that splitter to the cables going to various rooms in your house/apartment.
At the room where you'll set up the Roamio, connect the coax to another 2Ghz splitter, and run one output of that splitter to your cable modem, and other output to the Roamio. Note: If you have a central cable junction box don't put the cable modem and router there; TiVo tech support told me you need to have them physically near the Roamio, so only a short high quality (cat6) Ethernet segment separates cable modem and Roamio. I don't expect that really matters if you're using MoCa (rather than Ethernet) to connect between the Roamio and Mini, but in the off chance it might I did as he said.
Connect Cat6 (not cheaper Cat5) Ethernet from cable modem to WiFi router, and another Cat6 Ethernet from WiFi router to Roamio. Again I don't know how much using Cat6 matters when you'll be sending the video over the MoCa network, but might as well use the higher grade cable to be more "future proof".
On the Roamio DON'T try to go to Settings->Network->Connect to MoCa Network as described in the setup instructions. You only would do that if you're using an external MoCa adapter, but the Roamio and Mini both have built-in MoCa adapters. Once you've hooked up both the Ethernet and the coax cable to the Roamio as described above, the Roamio will be in MoCa bridge mode (bridging the Ethernet connection from the Internet into your coax MoCa network). Note: this step is where I wasted most of my time with tech support. We kept trying to get the Roamio to "connect to MoCa connection", and it kept failing with error code C33. I luckily finally reached a tech support representative who knew that was the wrong thing to do.
There's also no need to go through the setting to allow remove devices to control the TiVo because the TiVo software update is now out that supports dynamic tuner allocation (which then lets remote TiVo minis use a tuner to watch shows on the Roamio or watch live TV).
At the TiVo Mini go to Settings->Network and select MoCa network. It should find the Roamio and let you connect and watch live TV or shows on the Roamio.
Note also that when you move cable cards between TiVo boxes you need to re-activate your premium channels with the cable company. Call their tech support, ask to work with someone on Cable Card pairing, and they'll ask you to bring up Tivo settings screen for cable card pairing, and read off the cable card serial number, host ID and data.