Initial Set-up of AM21 Off Air Tuner with HR22-100

I recently purchased an AM21 Off Air Tuner and hooked up to my HR22-100 with an inside antenna attached.  When I went to Satellite Set-up in the menu and re-set it up, no Off Air Stations came up in the guide.  Nowhere in the set-up process did it ask for me to put in my zip code. When i called Directv customer service, they told me the antenna wasn't strong enough to pick up off air stations, so the HR22 receiver never searches for them.  I'm skeptical of this because we live less than 5 miles from one of our local (NBC) stations broadcast antenna.  Am I missing a step in the set-up process or is that something else I should do?  I wanted to try the small inside antenna to confirm the AM21 would work before I went to the time and expense of hooking up a large, attic antenna.