The documentation and your experience suggest that R2 is not supported, so you will need to resolve the problem some other way.
I thought that it was working for me, but didnt’ test very carefully and haven’t used it beyond a trivial smoke test.
If the backup wont’ download the files from R2 then you’ll need to use some other tool to retreive the files from R2 and then push them to your new bucket and then update the database by hand (you can search for topics using the remap rake task, I think).
This is a complex process, so you’ll need to understand a good deal about R2, S3, and Discourse’s Uploads. It’ll likely be difficult to explain your exact issues with enough specificity that you can get a solution on a forum.