Try using these DTB options.
I used the rk3399-rockpro64.dtb and it works.
One question, what is the source of your build? Is it a nightly from the LibreElec Github or what it "underneath"?
Short info about HEVC: Videos with HW acceleration are not shown (only sound) and without HW acceralation the playback is not smooth.
H.264 videos work without problem.
I think the community is still working on the HEVC support for the Hantro drivers in the 5.x kernels. I saw some posts in the more recent kernels, but I don't know the details or progress