after a factory reset, because RMS connection was not possible anymore and the WIFI was not visible, the router is not accessible anymore by any admin credentials and an update is not possible.
During the update process following error message is shown.
"Something went wrong during update. Probably you have chosen wrong file or you were trying to update ART on device with unknown FLASH type which is not allowed. Please, try again or contact with the author of this modification. You can also get more information during update in U-Boot console."
Thanks for your support