I have I wireless Rokus (both Streaming + and Ultra), wired power + HDMI connection that can't stream 4k and/or HDR to my LG c7p as the screen goes black (maybe a flash or 2). When I attach either of these devices to my old LG 55 (non 4k/HDR), they work fine. The LG c7p works fine with 4k/HDR from Directv and/or WEBOS Netflix connected to the same HDMI ports.
The TV is set to Deep Color mode and the Rokus are set to 'detect automatically' (though the same results when set to 4k 30 or 60 FPS). The menu will come up and <4k content will work fine, but whether Netflix or Amazon or Youtube, once 4k/HDR starts streaming to the LG c7p, the screen stays black and the 'no input dectected' LG screen appears. It 'appears' to be some kind of 'resolution negotiation issue' and I've tried, unsuccessfully on all 4 HDMI inputs. I've also checked for updates and both the Rokus and LG are up-to-date.
Re: Roku 4k/HDR content streaming crashes on LG c7p
I have an older post that explains this - at least on some LG TVs. The "fix" is to set the HDR "subsampling" to 4:2:0 vs. the default 4:2:2 that normally comes up. When set to 4:2:0, you should be able to get 4K HDR.
Go into this menu:
System -> Advanced System Settings -> Advanced display settings -> HDR subsampling settings
It's probably set now for 4:2:2. Change it to 4:2:0. Try your 4K HDR viewing afterward.