I have a couple of the "Battery Backup" devices that AT&T used to furnish with their Uverse gateways.
They just plugged into the power input jack of the gateway, then plugged into the wall. Inside was a standard 12 volt burglar alarm battery. It was about the size of a small lunchbox.
These things have turned up at garage sales, thrift shops, etc. Whether the power plug matches the jack on your gateway I'm not sure, you might be able to find a plug & rewire it to match your gateway.
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