It works fine now (obviously) but without that DECA dongle on the HR21 it is worthless at this point. BTW I did have On Demand working by just running wired ethernet to my router at one point but I knew that was not a long-term solution as I wanted whole-home and such, and would need to use DECA networking at some point.
If the Ethernet wire to your router is still there, just plug it in to your HR21, and you don't need the decca adapter for that box. As long as the home cinema kit and HR21 ethernet are run to the same router, they will see each other. Ethernet is a better connection anyway.