It's still possible to set up port forwarding on your BT Home Hub 5 to view the NS801 system on your iPhone whilst you are away. This is required, no matter what app you use to view the cameras.
Here are the instructions:
Have you tried accessing by any other app or by a web browser?
You can ignore the automated offer for remote fix, as I don't think we will be able to resolve this remotely as you have already set up the port forwarding.
You don't need a static IP address to access your NVR system remotely.
You can try some of these apps:
Have you viewed the software manuals beside each app on the link I provided?