your AMD/Intel GPUs are not currently supported by Catalina Metal driver, but they will be supported with the upcoming Mac Os 11 Big Sur new Metal driver.
OctaneX PR2 is already working with Big Sur beta Metal driver:viewtopic.php?f=33&t=75727
So, as soon as Apple will publish Big Sur, your Mac will be supported.
Alternatively, you could add one or more of the currently supported AMD GPUs in eGPU box connected via Thunderbolt 3, like Radeon VII or 5700 XT. in order to be able to use OctaneX on your Mac now.
I’m using this solution with my iMac with AMD 575 GPU.