Wireless streaming is very "your mileage may vary." You could try it. It might work, or it might get a lot of buffering or not work at all.
If the latter, the Roamio will have to be wired in some way too. If not ethernet, then a couple of moca adapters will be necessary for the basic Roamio.
The Mini has moca built-in so if you do create a moca network and have coax at the Mini's location already, it won't need ethernet.
The below picture shows how a moca network is created by connecting a moca adapter to the router/modem. It's not too complicated.
Then if ethernet is not desirable at the Roamio, a second moca adapter can be used to add the Roamio.