Cant view cameras on app when not on home network. How to set up router?

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My home CCTV system consists of a RLN8-410 NVR, 4 RLC-810A cameras plus a Duo wifi camera.

All work perfectly well, but when viewing through the Reolink app I am only able to view all the cameras while my phone is connected to the home network. When I am away from home I am unable to view the cameras connected to the NVR, though I am still able to view the wifi camera.

I have recently changed my broadband provider though I have always had the same issue. I presume that there is some setup required to allow the NVR to be reached by the app through the firewall. I have followed some guides that I have found, both from Reolink and from the router manufacturer (Fritz!box 7530AX) but so fat I have not been able to get it working. Does anyone have any suggestions?

A secondly issue that has only occurred sing I change my broadband is that occasionally the wifi camera shows as not connected, even when on my home network. Previously I had by wifi set up with separate 2.4Ghz and 5Ghz networks, but have left them combined wih the new router. Could this be a cause of the issue, do I need to separate the different bands to ensure that both the wifi camera and my phone are connected tot he same network for it to work?