AVM devices start a limited ftpd for a short time from the bootloader (ADAM2), which is meant for their windows based recovery programs, but can also be entered with cli ftp clients. This access method is used for flashing OpenWrt on these devices - and yes, it's really (a subset of) ftp, not tftp.
Edit: The tricks are:
- to hit the right time for invoking the ftp session, as there's only a very small time frame to catch (1-2s during the early boot process, while the bootloader/ ADAM2 is in charge).
- to only use passive ftp (if your ftp client allows setting this via cli parameter that might be better/ quicker than invoking
pasv at runtime).
- to provide the login credentials (adam2/ adam2) as quickly as possible, as only an established ftp session (login succeeded) intercepts the normal boot process.
it will usually require several attempts to hit the right timing, adding an unmanaged switch between your router and client can help to reduce the time needed for link training.
Once your ftp session is active, you can switch to
binary mode, invoke the custom
quote MEDIA FLSH command and
put the required firmware components directly to the relevant
 this ftp window is open during every single boot, it doesn't require pressing any buttons (there are no reset buttons)