Hi,
I had a same issue with my old DM500 HD... And if you don't have an IP you will need to connect directly to your STB.
Connect your DM500 HD (service port) to your PC with a USB cable and try to flash with DreamUp 1_3_3_12 (or newer)
1. start dreamUp (box is switched off)
2. Via menu Extras/(list of boxes with DM500HD included)/recover bad sectors is activated
3. push on the connect command button
4. switched on the power (at the back).
5. the flash-loader is uploaded