You can also try the same addon in AndroidTV firmware with Kodi
Maybe Plex will work better in other Linux OS’s when all Rockchip components are added to mainline Linux
You can try mainline Manjaro but only H264 codec is working so far, H265, VP9 will be available in a few weeks. https://forum.manjaro.org/t/manjaro-arm-20-04-released/133374/157
(Remember to erase the emmc to boot this Linux from a micro-sd card)