Update... since this older Roku Ultra also had audio lag only with Dolby Digital 5.1... I decided to buy the new Roku Ultra 2020. This fixed my remote problems, the audio lag issues & HDR. I would recommend the new device to anybody with any of these problems.
I also just bought a new Samsung and the Roku remotes sound buttons also didn’t work. After a bit of poking around on the TV settings, I went back to Roku settings menu and found that I needed to pair the remotes with the new TV! Duh 🙄. Problem solved and now I can enjoy my brand new 4K TV. 👍