If the device fails to boot after install or upgrade, whilst the unit is turned on:
Wait 15 seconds
Switch Off and Wait 10 seconds
Repeat steps 1 to 3, 3 times then go to 5.
U-boot will have now erased the failed update and switched back to the last working firmware - you should be able to access your router on LAN.
The steps for manual recovery are the same as the generic u-boot tftp client method.
Back To Stock:
Use the generic recovery using the tftp client method to flash the “civic.img”. Also you can strip-and-pad the original image and use the generic “mtd” method by flashing over the “kernel” partition.
Just be careful to flash in the partition that the device is currently booted.