So, you have downloaded the correct image, you have downloaded the correct device tree.
You then created the image using? (I use rufus) you then copied the device tree to the root of the sd card, renaming it to dtb.img
You then plugged in the microsd card, while holding the reset button, you plugged the power lead in, & release the reset button after a few seconds.
Yes?
Yes, I downloaded LibreELEC-S905.arm-8.0-8.0.2a.img.gz and gxbb_p200_2G_100M.dtb (tried with gxbb_p200_2G_m8s+_k3_pro.dtb too). Then I created the image using rufus and replaced renamed device tree. Next I plugged-in USB pendriver, the power supply cable was disconnected and I held the reset button pressed until the boot screen shown above appeared.