Roku voice remote has not controlling power and volume for NIKAI UHD 55 inch Tv . How to solve this?

Thank you for letting us know, Mahmood.

In addition to my question above, where are you currently located? Also, is this your first time setting up your Roku Ultra? 

In the meantime, you can press and hold the back and home buttons simultaneously for about 30 seconds to reset it and then re-pair right after. 

Let us know if this resolves your concern and we'll go from there. We'll be waiting!

Best regards,
Carly