Try to find a tossed out rack mount UPS, which will very likely just need new batteries, but is tossed out because the typical rack user will only buy OEM, which works out at being around the price of a new one. Got 2 of them for free from a local IT company, just ask them if they have some around, you will get typically IEC connectors on the input and output, though some have high current versions, so you will need to buy the rewireable cable sockets to use them. Using an APC 750XL at the moment, with the batteries external to it, as the ones it came with were stone dead, and I wanted more run time, so 2 60Ah car batteries are now plugged into the external battery socket.
The 2 rack mount units, one Eaton and one APC, probably work perfectly, just that I have never put new batteries in them, as they need 36 or 48V to operate, and I did not have the spare SLA batteries around to test them, the internal cells were well cooked. Heavy units, even without the batteries, so would probably work well and give you more than you need.
With your networking equipment it likely will run fine off 12VDC, so a simple UPS would be to have a regulated battery charger and a 12V battery, and connect the equipment to this instead. Easiest way to do this is to look for a security system power supply, which has a power supply that charges a SLA cell, and also provides operating current for the cameras and DVR from the battery. Lowest losses, though the charger/power supply will lose around 5w in operation providing the regular load.