If you find all cameras' image disappears on TV frequently, please try methods below.
1. Please unplug your NVR from the power source and plug it into power source again to reboot the system.
This method is effective to solve the image disappearance problem.
2. Please disconnect NVR from the router to see if the problem can be solved.
3. When the camera's image is not shown on TV, please go to the setting to see if NVR can detect the camera's IP. If yes, you add a camera to NVR manually to show the image.
Here is the instruction: https://support.sannce.com/hc/en-us/articles/900000451086
4. Please go to the Main Menu-->Advanced-->Restore to select all to restore the system to have a try.
If none of the above methods work, please contact firstname.lastname@example.org for further help.