The monitor's or TV's resolution must be higher than the recorder's display resolution. If not, monitor or TV is not able to show any image.
If the monitor or TV can't show any image at all and says not supported, it's maybe the recorder is higher than your monitor's, which causes no image.
There are 3 ways to solve this issue.
Method 1: Please use a higher resolution monitor to connect your recorder so that you can log in recorder and change its display resolution to the lower one.
Method 2: Please access the camera with software CMS on a windows computer to change its display resolution to the lower one.
You can refer to this link below to access the system on CMS.
You can refer to this picture below to change the resolution on CMS software.
Method 3: You can access the recorder system on the computer browser by the recorder's IP and change its display resolution on the web page. You can refer to this link below to find its IP and access it.