In Mac OS Mojave, I’m told you can select a program and set it to “prefer external graphics”.
That may get it to use the eGPU for the Blender viewport and Eevee if it isn’t already.
However I believe the only AMD GPU compute option for MacOS is OpenCL, and Blender dropped support for OpenCL on MacOS in December based on Apple deprecating it and issues that were making it difficult to support. So you may be prevented from using GPU rendering in 2.80.
Try an old 2.79 build like official 2.79b (not the current experimental 2.79 build which uses the same Cycles version as 2.80) and see if that recognizes the device.