@PriyankP Can you remove and re-add the device to your app by scanning the QR code on the NVR itself? If doing this while your phone is on the same LAN as the NVR, you should be able to add the NVR to your devices and view it locally instead of over the internet. 

@NeilR_M I tested the suggested steps and here are the results.
 

Scenario 1
Phone connected to the CCTV VLAN via WiFi. Cellular data OFF.

I cannot add the device by scanning the QR code. The app reports that the device is not connected to the internet.
The NVR does appear in the auto-discovery list, which confirms the phone can see it on the local network, but adding it still fails with the same "no internet" error.
 

Scenario 2
Phone connected to the CCTV VLAN via WiFi. Cellular data ON.

In this case the app allows me to add the NVR. It shows recording thumbnails, but playback does not work and live view does not start.

My expectation is that internet access should not be required in either scenario. If the app can discover the NVR locally or if I provide the local IP address, the connection should remain entirely within the LAN. Requiring an internet lookup in this situation does not seem necessary.
 

Internet access might make sense when adding a device using a serial number or QR code as ID to IP lookup table may be hosted in the cloud. However, when the device is clearly discoverable and reachable on the local network, the dependency on internet connectivity becomes a significant security and privacy concern.