I’ve recovered three v2 hubs with the below procedure:
If you are able to factory reset it, remove the batteries, and Unplug Ethernet and leave it unplugged. Perform the factory reset (press and hold reset button while plugging in power, keep holding until flashing yellow LED goes yellow solid, then release the reset button)
Let the hub reboot and go through the motions and LED cycle allowing to boot and stay up for about 5 minutes (continue to keep Ethernet unplugged). You’ll see the led cycle red to magenta to flashing blue, etc.
after 5 minutes on flashing blue, power cycle the hub one more time by unplugging the hub and then plugging it back in (again, keep Ethernet unplugged) and allow the hub to boot.
then let it sit for 5 minutes without Ethernet. The LED will either be a flashing blue or solid blue at this point but after 5 minutes, now plug in Ethernet.
within about 1-2 minutes after finally plugging in Ethernet , you should see the LED go green and then it should pretty quickly go magenta to indicate it is download the firmware… don’t touch the hub at this point, just let it cook as it will do this cycle one more time before it finally gets upgraded to 57.x.