Have a look at this folder (collected by FOV):
https://onedrive.live.com/?authkey=%...73C0EB037CE1E2
In the folder:
RK30SDK_T6x
you will find:
T6x_174_Gonyx_S512mD1g.7z
which will probably work with your device, but make sure to follow the instruction otherwise bricking is possible.
The other possibility is using the ROM Stangri has found:
https://www.mobileread.com/forums/sh...&postcount=213
Good luck!