The monitor‘s resolution must be higher than the display resolution of the DVR. If not, the monitor is not able to show any image of the cameras.
If the monitor can‘t show any image at all and says not supported, it can prove the display resolution of DVR is higher than your monitor‘s, which causes no image.
There are 4 ways to solve such kind of issue.
Method 1. Use a higher resolution monitor to connect DVR so that you can log in to DVR and change its display resolution back to a lower one.
Method 2. Access camera via software CMS on a windows computer to change its display resolution to a lower one.
Here is a method to access the system on CMS.
Here is a place to change the resolution on CMS software.
Method 3. You can access the DVR system on a computer browser by DVR‘s IP and change its display resolution on a web page. Here is the method how to find its IP and access it