You can update firmware from that page, but not load custom firmware, or firmware meant for something else from that page, most likely. Most device config pages can be accessed by the IP, but will only update firmware meant for the device from there. Depending on what the firmware and hardware actually is, it may or may not be illegal to mess with it.
You should try and look up the firmware on the internet, or maybe telnet into the box and see if you can determine what the firmware actually is and see if it's even something you can legally mess with. Or look up patents from the manufacturer, etc, etc. If it's something open source and not encrypted, not password protected, it might be legal to mess with, then maybe with a lot of effort you could do something, but very likely it's not worth it, you'd most likely need to build a new firmware yourself and be proficient in coding for the processor in the unit, memory, bus, etc, or do a stupendous amount of work.
If it's proprietary and closed source firmware, password protected or encrypted, walk away from it and don't mess with it. Even just using the hardware with different firmware could possibly be a issue, it may not be legal to flash different firmware into it, depending on what it is.
If you really want to pursue this, then search and read on the internet about what you want to do, without a box like yours physically in front of us there's no way to tell if it's legal to mess with it and hack stuff isn't allowed here, so it's kinda of a grey area, what you're trying to do, on here. My guess would be that it's proprietary, but that's just a guess.