Its very possible that port 80 is blocked inbound to the house. ISP's sometimes do this so that you don't setup a webserver without paying for a commercial internet package.
Can you change the port to anything else? Can you access it internally? If so, then your issue lies somewhere with router>modem>ISP. A lot of modems have firewalls integrated these days, so you'd need to forward the ports from the ISP's device to your router, then your router to the DVR, etc.
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Mokli - Druid of Karana
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