Please think twice and realize people can not help you when they do NOT know what the particular device are you talking about!
What the particular model of the Defender DVR device you are talking about?
Here is an article about hardcoded support passwords in some DVR's including some Defender DVR's (a brand of Circus World Displays), which (may be!) you can use to access your device.
Read carefully and look if this info related to your case.
Search with Google info on your particular model, it's internals and access methods. Most of advanced network DVR's contain Linux-based firmware, so there are good chances for hackers to explore firmware internals and find useful things.
In the modern world any devices may be produced by any manufacturers and sold under any brands. Thus, internal architecture of the particular device and its real properties is highly depend not only on the particular device model, but even its hardware revision. E.g. many network devices having the same brand, the same model, the same casing, but JUST ONE different letter in the revision string could have been made by absolutely different real manufacturers and have absolutely different boards, CPU, hardware configuration and firmware.
It's a real fraudlent world and stupld 'money making Matrix system', we built. Forget about brands. The only countermeasure we have is our wide knowledge and accuracy.