Need help to debrick Netgear DGN3500

My DGN3500 became brick after upgrade.
When updating the firmware, I used Factory image. Wrong.
Now the router does not boot. I really need help.

I don't know the device. On device page there are no device specific hints for tftp flash. But there is also a generic tftp guide available:

As I assume your device' bootloader contains a tftp server so you have to aim for this section. Read it carefully and prepare everything (maybe double check things if you don't have experience). If you have questions ask here me and maybe others will try to help.

There is also a tool called nmrpflash for netgear routers available:

It is much easier but for this method there is no warranty that it will work with your specific device (compare device list). Maybe it works.

To get a feeling about the process you could watch a video on youtube. Just search for "router recovery tftp".