I updated the firmware and when it restarted all 4 LED lights were just flashing green.

Resetting the device did not help.

This is what I did to fix it:

Connect a network cable from the extender to your computer.

Set your computer network card to IP/Subnet to 192.168.0.111 (or similar unused IP) and 255.255.255.0.

Download an IP scanner, I used free PingWS.

Scan the 192.168.0.* range to see if it can find the extender.

It found the faulty extender on 192.168.0.254.

You can manually just try ping this address and the other 253, but it might take time.

Open 192.168.0.254 (or whatever yours is) in a web browser.

Log into the extender with the default password = admin (if you have reset the extender).

Download a working firmware version from the TP-Link website.

Upload the new firmware from the extender control page within the browser.

After these steps my extender was back to normal working order.