The monitor's resolution must be higher than the display resolution of DVR.If not, monitor is not able to show any image of cameras.
If 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.
The DVR output display minimum resolution is 1024*768.
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 DVR and change its display resolution back to a lower one.
Method 2. Connect DVR to a monitor via VGA port instead of HDMI port so that you can eneter system and change its display resolution back to a lower one.
Method 3. Access camera via software CMS on windows computer to change its display resolution to a lower one.
Here is method to access system on CMS.
Here is place to change resolution on CMS software.
Method 4. You can access DVR system on computer browser by DVR's IP and change its display resolution in web page. Here is mthod how to find its IP and access it