The first step is to make sure the NVR / DVR IP Address never changes. Ensure that your NVR is set up with either a Static IP Address, outside of your router's DHCP range, or that you have configured an "IP Reservation" for the NVR.
If that's fine, we'll need to double check your port forwarding rules. A common mistake many make when port forwarding is, create the rule in this structure: "TCP 80 --> Port 80 of NVR" when it is supposed to be structured like "TCP Any --> Port 80 of NVR". If the beginning of your port forwarding rule isn't set to use TCP / UDP with "Any," the rule won't work as expected.
After double checking these two points, use the website http://www.canyouseeme.org/ to check to see whether or not your NVR is reachable from outside. It will report "Open" if the rules have worked. "Closed" means the port forwarding rule is likely working, but the NVR is refusing connection. "Stealth" means the port forwarding rule is likely to be incorrect, or the NVR is offline entirely.
The last part to ensure is, How are you trying to connect to your NVR remotely? By IP Address, a Dynamic DNS name, or is there a cloud connection the NVR makes to keep track of your NVR's public IP?
