No 5.1 when plugged into LG TV

I have an LG 4K TV (65UF9500) and can't get 5.1 audio options from either of my Rokus (4K+ & Express), only stereo. I get 5.1 from either Roku when plugged into my AVR, but then I can't get 4K because it's limited to 1080p. 

I've spent hours and tried each of the many fixes I found, but to no avail. I think it's possible that it has something to do with the TV only having Dolby Digital (not Dolby Digital Plus) and the EDID limiting the Roku to stereo. Does this sound right? Is there a fix?