I have a RaspberryPi at home running PiVPN (OpenVPN) and a VPS connected to the VPN as client. VPS works fine as VPN client, I see network devices, I can ping to Internet, etc.
I use this VPS as video surveillance system (Shinobi software) which connects to my home network cameras and it has a web panel to manage the system running on port 8080.
So, the problem is that if I access to my VPS web admin url (http://VPS-PUBLIC-IP:8080) in my home network/VPN I can access to the web panel with no problem, but if I try it outside my network, for example with my mobile 4G network, the port is closed.
I tried to open port 8080 in my router for RaspberryPi IP (192.168.1.8) and VPS client IP when is connected to the VPN (10.8.0.2) but I had the same result.
As you can see, I am not an expert in Linux networks.