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.
There are 3 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 lower one.
Method 2. If you have added system before and know its SN, here is method how to connect coputer brower:
Use a computer to log in DVR and change its display resolution back to lower one.
Method 3. You can access DVR system on computer browser by DVR's IP and change its display resolution in web page. Here is method how to find its IP and access it: