Reset Sophos AP55 with openWrt installed


i have a problem with my Sophos AP55, there is already OpenWrt installed but the User & Password is unkwown. This Access Point has no Reset Button and i dont know how to reset to factory defaults. Can anyone tell me how i can reset this AP or reflash the Firmware?
I tried to reflash the original sophos firmware but the Sophos reflash tool didnt work, because the fallback ip is different now.

Did you try the tftp method: