I feel as though I've tried everything, from complete uninstallation, wiping, restoring old drivers and so on. Every time I get the same message:
Radeon Software (220.127.116.11) has not been installed. Error 1603 - A general error has occurred during the installation of the AMD software package.
When I go to uninstall or retry the installation the AMD client says that the drivers are 'partially installed'.
This is really annoying. Using two monitors is extremely problematic now as for some reason my mouse keeps lagging.
I'm using a 21" 4K iMac (2017) running the latest version of Windows 10 on Bootcamp.
You would contact Apple Support for anything on your Mac, including running WIndows on your Apple product. AMD does off the drivers here as a courtesy but they are the same as the ones on the bootcamp page and the support is through Apple.
I am sure some user knows how to help they will but these forums are not AMD support they are USER TO USER support. So very few would have Mac experience so I want you to get the help you need and that is likely from Apple Support or an Apple Forum.