Please get a monitor with VGA port if you can. Then please connect DVR to monitor with VGA cable to test if you can see video on the monitor.
If you can see live video on monitor via VGA cable, enter into main menu---system--display and change resolution to lower one. Finally, you can find HD TV shows live video.
Please check whether computer can detect DVR's IP and access system.
If it works, please change display resolution to lower one. Finally, you can find HD TV shows live video.