If you get a gigaset DE900 stuck in rescue mode, this is what to do.
First change your computer to have an IP address of 192.168.15.2 with a 255.255.255.0 netmask. The phone will be on 192.168.15.1
Download the Firmware file
Open a “DOS-box”
go to the DIR where the Firmware is located
TFTP -i 192.168.15.1 PUT filename.IMG
Open a Terminal (Firmware file has to be in users home-DIR)