In the NVRs Advanced Storage settings you need to make sure you have Enable Overwriting unticked, that way once the storage is full it will not record over any of the old footage. If the cameras are disconnected then no new footage should be recorded that would overwrite the old footage but just to be safe we would untick this setting.
If you go to Storage > Schedule Settings > Record Schedule and click the advanced button in the top right of the schedule you will see the below pop-up:
Here you can set it to record a certain number of days, but you have to combine this with the overwrite setting because if for example you set this to 14 days and you have not disabled overwrite it will record to the 14 day limit and then start overwriting footage from day one. Also, be aware that you have to set the number of days for each camera connected to the NVR, this settings does not automatically apply to all cameras. You can either manually go through and enter the number of days one camera at a time, but if you have full NVR you may want to use the "Copy to..." button at the bottom left of the schedule to copy the number of days to all cameras.