I'm having the same issue too.
Even after following the instructions on the guide linked by @timonsku, my Unifi 6 Lite just boots back up in adoption mode (solid white LED) with the stock firmware seemingly intact.
➜ ~ scp -O ~/Downloads/openwrt-22.03.5-ramips-mt7621-ubnt_unifi-6-lite-squashfs-sysupgrade.bin ubnt@192.168.1.20:~
ubnt@192.168.1.20's password:
openwrt-22.03.5-ramips-mt7621-ubnt_unifi-6-lite-squashfs-sysupgrade.bin 100% 6913KB 5.3MB/s 00:01
➜ ~ ssh ubnt@192.168.1.20
ubnt@192.168.1.20's password:
BusyBox v1.25.1 () built-in shell (ash)
___ ___ .__________.__
| | |____ |__\_ ____/__|
| | / \| || __) | | (c) 2010-2022
| | | | \ || \ | | Ubiquiti Inc.
|______|___| /__||__/ |__|
|_/ https://www.ui.com
Welcome to UniFi U6-Lite!
********************************* NOTICE **********************************
* By logging in to, accessing, or using any Ubiquiti product, you are *
* signifying that you have read our Terms of Service (ToS) and End User *
* License Agreement (EULA), understand their terms, and agree to be *
* fully bound to them. The use of SSH (Secure Shell) can potentially *
* harm Ubiquiti devices and result in lost access to them and their data. *
* By proceeding, you acknowledge that the use of SSH to modify device(s) *
* outside of their normal operational scope, or in any manner *
* inconsistent with the ToS or EULA, will permanently and irrevocably *
* void any applicable warranty. *
***************************************************************************
U6-Lite-BZ.6.2.44# fw_setenv devmode TRUE
U6-Lite-BZ.6.2.44# fw_setenv boot_openwrt "fdt addr \$(fdtcontroladdr); fdt rm /signature; bootubnt"
U6-Lite-BZ.6.2.44# fw_printenv
appinitdone=true
baudrate=115200
ble_mode=serial
bootdelay=2
device_model=U6-LITE
ethaddr=d0:21:f9:42:25:78
fdtcontroladdr=8ffdd200
fw_version=9.9.9
ipaddr=192.168.1.20
is_ble_stp=true
macaddr=d0:21:f9:42:25:78
netmask=255.255.255.0
serverip=192.168.1.1
stderr=uartlite0@1e000c00
stdin=uartlite0@1e000c00
stdout=uartlite0@1e000c00
ubntaddr=850000a0
is_default=true
bootcmd=run boot_openwrt
devmode=TRUE
boot_openwrt=fdt addr $(fdtcontroladdr); fdt rm /signature; bootubnt
U6-Lite-BZ.6.2.44# ls
cfg
openwrt-22.03.5-ramips-mt7621-ubnt_unifi-6-lite-squashfs-sysupgrade.bin
U6-Lite-BZ.6.2.44# cat /proc/mtd
dev: size erasesize name
mtd0: 00060000 00010000 "u-boot"
mtd1: 00010000 00010000 "u-boot-env"
mtd2: 00040000 00010000 "Factory"
mtd3: 00010000 00010000 "EEPROM"
mtd4: 00010000 00010000 "bs"
mtd5: 00100000 00010000 "cfg"
mtd6: 00f10000 00010000 "kernel0"
mtd7: 00f10000 00010000 "kernel1"
U6-Lite-BZ.6.2.44# dd if=/dev/zero bs=1 count=1 of=/dev/mtdblock4
1+0 records in
1+0 records out
U6-Lite-BZ.6.2.44# dd if=openwrt-22.03.5-ramips-mt7621-ubnt_unifi-6-lite-squashfs-sysupgrade.bin of=/dev/mtdblock6
13825+1 records in
13825+1 records out
U6-Lite-BZ.6.2.44# dd if=openwrt-22.03.5-ramips-mt7621-ubnt_unifi-6-lite-squashfs-sysupgrade.bin of=/dev/mtdblock7
13825+1 records in
13825+1 records out
U6-Lite-BZ.6.2.44# reboot
U6-Lite-BZ.6.2.44# Connection to 192.168.1.20 closed by remote host.
Connection to 192.168.1.20 closed.
➜ ~