CCTV only connects remotely on local network not on public or mobile data.

I'm running into an issue where I've installed CCTV for my home, I have an app on my phone that is linked to the DVR which is connected to the internet using ethernet.

I used to able to view it from anywhere but now I simply can't. I can only view it if I'm on the same network as the CCTV itself, which is at home.

What could be the issue? It didn't have this problem before.