If you already have internet service through your local cable provider, you may already have some basic cable channels active. Try hooking a cable to a nearby tv from the wall and do a channel scan and see what you get. Sometimes having limited and/or basic cable along with your internet service costs about as much as internet alone, then you can rent a cablecard ($0 to maybe $10/month depending on provider) for your Roamio.
Like they said, the only way it's going to work is with cable, not DirectTV.
Director, Event Technology