Hey there serge, i hope you see this. I busted my head over this but here is a good firmware for the astak mentor (Netronix hardware):
https://astak.sharefile.com/?cmd=d&id=35bd1e05c34a43bb
and here are the instructions:
After download, unzip the rar file and put all extracted files to SD card root folder.
1. Put an SD card with all the update files in the the reader.
2. Remove the battery completely from the device.
3. Press and hold the Enter button and "Go Back" function button (third button on the left side).
4. Plug the device to wall while holding down two buttons.
5. When you see the update message, let go two buttons, and follow the instruction on screen.