If you can't add a channel to your Roku, it's likely it's not officially available in your location. Since your model number ends in EU, that means it was sold in Europe. Roku is only officially supported in three locations in Europe: UK, France and Germany. The channel developers (not Roku themselves) control where their channel can be installed, and Roku cannot override those settings. So Amazon is in control of where their Prime Video channel can be installed.
Remember, just because Prime Video (or any other provider) is available in your specific country, that doesn't mean the provider has made the channel available there on Roku devices. If you call Amazon and they say Prime Video is available in your location, that doesn't mean Amazon has made it available on Roku. And the customer service people you talk to simply don't know that. All they have are scripts that tell them what to say, and they don't have a script to address this specific issue.
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, just another user.