More info could help, as you can imagine there's many, MANY variables and possibilities:
- Brand/model of NVR?
- Name of app?
- Did you scan a QR code with smartphone or input a UID number?
- Has it worked before but recently quit?
By "External IP", assume you mean you have a public WAN IP. What happens when you're on your Wi-Fi and you use your LAN IP, not the WAN IP?
Does the app have a specific place (entry) for both?