Unfixable problem with Bricked Unifi 6 LR

I was trying to install openwrt on my new Unifi 6 LR, but encounter a strange problem. I was following the instructions here: https://openwrt.org/toh/ubiquiti/unifi_6_lr . The first time I tried it I used the v2 image, and after rebooting the AP eventually got to a blue light, but I couldn't find any way to connect to it, neither by trying a default ipv4 address like 192.168.1.1 or 192.168.1.20, nor a ipv6 link local address (it wasn't responding to ipv6 pings on ff02::1). it seems the ethernet pork didn't work. After that I was able to hold the reset button, use the tftp mode to reload the stock firmware, and follow the instructions again, this time using the v2 image. However, now whenever I power it on it immediately sets the led to white, and nothing else seems to happen. I still cannot connect to it, and holding the reset button seems to do nothing. I opened up the device and I could not connect to the device via serial ports. it seemed nothing booted. Does any one has some ideas which can help me?

This sounds like the bootloader has been clobbered by writing to the wrong mtd partition. Recovery is going to require hardware writing directly to flash.

Thank you so much for your help. I'm not familiar with this way and I don't have any idea about writing firmware directly to Flash. Do you have any tutorial or guide for doing it?