Here's is easy method to Flash any Image on Dreambox (DM520, DM900 and DM920 etc), without Internet Connection. I explain here for your DM520
Start on your computer with step 1 - 3 below
1. Format USB memory stick with Fat-32 and set the label to DREAMFLASH
2. Copy image to USB Drive
3. Rename image " dreambox-image-dm520.tar.xz "
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From here continue with your reciver (dm520hd)
4 Insert USB to receiver
power on your receiver and touch and hold front touch panel within 500 ms to bring receiver into Rescue Mode
Note: If receiver is on red light then first power off your receiver from rocker switch.
Then power on your receiver and touch and hold front touch panel within 500 ms to bring receiver into Rescue Mode
There is no Dosplay on your box so I assume there will be some lights indicating that flashing is in progress.
But when it's done the box should reboot automatic. Be patient it can take up to 5 min