That likely isn't going to be the issue. Most online streaming providers, such as Vudu or Netflix, will only stream HDR content to a 4K display. It doesn't matter if the Roku itself is 4K. If the Roku output is set to 1080, that's all the streaming providers will send, and I'm not aware of any of them offering HDR with 1080 content.
Dan
Roku Community Streaming Expert
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