First of all NR51P6-16 is not any Hikvision Device, maybe any Chinese copy of hikvision!
Second you if port 8080 is open in nat and is available on NVR you should type Public IP:8080 in your browser not just IP, because if you type only IP will respond to port 80 which belong to router web page!
I got the same issues and my Hik cam was working when i set a port foward. Now the firewall rules are quite too hard and that don’t work anymore, but i just connect via vpn and uber safer than having a openport.