Newer wifi voice remotes don't come with single pairing button, instead use combination key sequence to put in pairing mode. Press and hold the Back and Home keys for 5 seconds and release it should put in pairing mode. You original Onn Roku Soundbar remote was an IR remote, no pairing needed. And of course I don't really know what remotes you actually purchased. If you went for the cheaper $9 and $14 remotes from Roku they are the wrong remotes, only $19.99, and the two $29.99 remotes are the ones you needed.
Edmund
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.