Did you ever resolve your Roamio Plus setup issues? We have the exact same configuration.
roamio plus (won't create Moca Network)
Tried direct connect ethernet to cable modem
Turned off cable amplifier
After an hour on the phone with tech support, they're sending me a replacement unit. We couldn't get the Roamio to setup a Moca network. They had me direct connect the ethernet to the cable modem, turn off any amplifiers on the cable line, and reset the Roamio. Nothing worked.
I really suspect that the issue is not the Roamio, but some strange issue caused by the cable modem (comcast). I guess the replacement unit will prove out that theory.
So,i'm curious if you ever had success based on all of the forum feedback.
Thanks - Howard