Yes, you can upload it into the /tmp folder with WinSCP or any FTP tool (Remember it's binary and not ascii file, if you need to set the transfer mode do it in "binary").

The wget you put above is wrong as it's not the full file, there is ... then you should have the full link, https://dreamboxupdate.com/opendreambox/...00-20190911.bin

Code:
cd /tmp
wget https://dreamboxupdate.com/opendreambox/2.5/stable/images/dm900/zImage-rescue-3.14-r0-dm900-20190911.bin
flash-rescue /tmp/zImage-rescue-3.14-r0-dm900-20190911.bin


Once in the /tmp, or by FTP/transfer to box or using wget to download it, you can then use the flash-rescue tool to flash it to the box.

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