Found an easy way to flash OpenWrt to the Fritzbox 4020/4040/7312/7320/7330/7360

[tomtom]

Looks like AVM has changed the timing... about a year ago my first flash succeeded using the "old" method.
I found another thread regarding flashing the FB4040.

Problem with flashing OpenWrt Fritzbox 4040

Found a trick. You need a Windows computer. From AVMs website ownload a recovery tool NOT intended for the 4040 model. Start the recovery process as described in the running app. It will not install the wrong firmware but will stop. But it won't close the connection to the 4040. So you can just kick in.

But it requires a windows machine.
Did you try this one ?

Cheers,
Thomas