Installing LinuxCNC on Debian 9

At this point I don't remember how I did it. I probably just ticked the box to allow proprietary drivers to install and then searched for radeon in the software list.

You are not talking about the same thing.
And there are no Radeon drives in the driver manager in Linux, only Nvidia.
Radeon drives have to be downloaded from the AMD website.
Nvidia can be made to work with Preempt-RT in some cases, not always. AMD is much harder to make work.
Did not try the RTAI version.