I have the Roku Express 3900X. When I try to setup Apple Homekit, I scan the QR code, and it adds just like any other item. But when I try to control anything about the Roku in the Home app or Siri, it says that there is no device in Home. The software is up to date, My tv and HDMI cable both support HDMI-CEC.
Check if the Roku shows up in AirPlay from different apps/Control Center mirroring from your phone/tablet.
If it doesnt, try rebooting/restarting all the devices: Roku+phone/tablet+modem/router/gateway
I have already tried every single one of those things 3 times, nothing different happened. Airplay works fine, but it is Homekit that is the issue.