Sagemcom aka plusnet 2704N V1 lost password

Hello, i got this device and openwrt worked fine.
but i simply lost the root password and luci web ui does not work anyway, neither the reset button.

So how can i reset configs in the easy way?
I tried to boot in failsafe mode by pressing a button when a red led blinks and then after that the led start to blink faster, but cannot access a host in, there is other subnet to try? i have not found info about this , if it is really a safe or recovery mode...
anyone have a clue in how to solve it without need of soldering?
table of hardware:

If doesn't work (it should), you will need serial console access (which presents a passwordless login shell).