After your device enters into Download mode, the button Edit Phone Information will be available. Push that button and wait for an error (something like failed to reboot to FTM mode) being appeared. Don’t be so tragical about it. Press OK and see the button Next should be activated.
After button, Next become active, press on it and have an eye on the flashing process or have a cup of tea not to kill time.
Your device will reboot and you may congratulate oneself.