It does help to know the model number of the unit. As for solving your issue, you would need to purchase a OTA add-on tuner for ATSC & QAM, such as this http://www.channelmaster.com/Digital_TV_and_HDTV_HD_digital_Antenna_Tuner_CM_7001_s/287.htm
Channel Master is just one company out there that makes these boxes. They do also offer a DVR model. The Silicon Dust models at http://www.newegg.com/Video-Devices-TV-Tuners/SubCategory/ID-47?Tpk=atsc%20tuner
are popular also. It is really how much are you willing to spend, and what extra features are you looking for.
Now of course you could get a refurb desktop or tower unit from say Tigerdirect.com and roll your own HTPC with a add-on ATSC tuner card, which would give you more options than most off the shelf manufactured OEM units out there.