The only Roku Channel APK I have found that worked (and still does) on our Fire devices came from TechDoctorUK’s website: Roku TV Official APP For Firestick / Android TV | TechDoctorUK
It’s commercial-free version 1.0 . Hope this helps! 
PS: I should probably mention that I’m in the US.
Tried on my tablet, but it won’t play. I’ll try on my nVidia in a bit.
If you do a search for ‘Roku Channel’ on TDUK’s site, the main Roku posts are from Oct 2020 (the link I posted), and there are two others more recent from mid to late 2021 that also have Roku Channel APK’s. I’m not sure if they are all for the same version. There are others I’ve seen around (and tried on our Fire Cube) that have longer version numbers, but the only version I have found to work reliably for us installs as Version 1.0 . I think that IP TVSuperStore’s Unlinked site may have had that 1.0 version available, too, IIRC. 