The cameras need a cloud connection as the metadata and information relating to stored video is in the cloud. Additionally, the UI is dashboard which is served from the cloud. Without access to this information and services, your client can't access a UI for the cameras, nor can the camera retrieve the metadata related to the video it has stored.
However, as the video would be streamed directly from the camera, a small backup link is sufficient to run many cameras e.g. LTE or cellular backup can run over 100 cameras if the video stream is going local.