Here is AMD ProRender site: https://www.amd.com/en/technologies/radeon-prorender
There doesn't seem to be a AMD ProRender Plugin for Cinema 4D:
Note: Last year ProRender Plugins revised the GPU list compatible with the ProRender Plugins by eliminating all Non-RX Series GPU Cards like the R9 Series GPU cards. Not sure if this is also the same for the regular ProRender app or just for the Plugins.
Here is AMD webpage concerning ProRender and Cinema 4D: https://www.amd.com/en/graphics/workstation-media-and-entertainment-solutions-media-maxon-cinema-4d
But you can always open a thread at AMD Forum's ProRender Plugin site and one of the Moderators can probably answer your question from here: https://community.amd.com/t5/radeon-prorender/ct-p/amd-radeon-prorender
@bsavery
EDIT:deleted thread from ProRender since I noticed you replied to that thread in February 2021.