Netgear WGR614 TFTP Recovery

I have a Netgear WGR614v10. Around August 2016, I tried to replace the factory firmware with lede-brcm47xx-mips74k-netgear-wgr614-v10_north-america-squashfs.chk. This failed.

When the device boots it sends TFTP requests for vmlinuz. Could this allow restoring a firmware? Where can I find a suitable vmlinuz file?

You can try with official recovery article

There are some people on dd-wrt forums that have dumped the firmware with tftp AND serial access to the board (this allows you to tell tftp to load whatever you want).
but there is no info on what are the commands to flash it after you transferred it, nor what is the pinout of the serial. It should be 4 pins as this is TTL serial, not RS232 serial.

By looking at the images here it seems the soldered pins are the TTL but you'd have to use a multimeter to see what is power pins so you can ingore them while you try connecting the TTL-usb dongle to the others until you get the connections right.

From what I read from both the dd-wrt forum and in hardware specs, it's probably not really worth it. 4MiB of flash (no customization) and older broadcom wifi (crappy or non-existent opensource drivers).