I think there's an error here. Let's see what a DNS is first.
https://en.wikipedia.org/wiki/Domain_Name_System
Now while a private network can have a DNS server this doesn't mean it will work from say the internet since your entries won't sync with the internet DNS servers.
For most home DVR CCTV systems I can skip port forwarding and place the DVR's IP into the DMZ entry.
Sorry for the TLA.
I am having trouble viewing my camara system from it's dns?
I have done this before I am stumped right now!!

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