First, look closely at the battery positions in the remote. Some Roku remotes alternate the battery direction, while others have both batteries face the same way. Make sure you're inserted your batteries the right way. Next, is this a remote with a headphone jack? If not, then it's a simple IR remote, and you can test it using any digital camera, such as the camera in your phone. If you look at the front of the remote through the camera and press different buttons on the remote, you should see a flashing light through the camera. No flashing light, the remote is not working. If you see the flashing light, then the remote is worked as expected, and the issue lies elsewhere.
If your Roku remote has a headphone jack, then it's a WiFi direct remote and the flashing light test won't work. But we need those questions answered before anything else can be suggested.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.