You can find the official firmware for the router by downloading the
Netgear software kit for the Sky DG834GT router. It's in "bzipped tar" format, so you might have some difficulty extracting the contents. The file you need is called
DG834GT-1SKUKS_V1.02.28.img. Note that that bzipped tar file contains a complete source code kit for the router, so it's more than ten times bigger than the image file, which is the only file you actually need from it.
Alternatively, you can download just the firmware image from
here, but that link is unofficial.