You can refer to these steps below.
1. Please go to Main menu---System---General to change video standard to have a try. If it doesn‘t work, change it back to the initial one.
2. Connect only one camera to DVR, because maybe power has some problem and can‘t support all working or there is signal interference between cameras.
3. Change a monitor or TV to check if it is a problem of monitor or TV, not DVR or camera.
4. Check if computer view or phone view face the same issue.
Here is a method to view on a computer.
Here is a method to view on the phone.
5. Change another adapter to have a try.