perhaps you need to close the laptop lid while you use TD?
On macOS you need to do 2 or 3 steps: enable eGPU for each app, set the main window to exist on one of the eGPU displays. Before Mojave you also had to close the laptop lid, but thankfully you no longer need to do this.
Also check egpu.io for a lot of good eGPU tips/guides: egpu.io
i was working on a notch project a while back using my macbook and an egpu (2080ti).
I was running the macbook in bootcamp and followed some instructions I found online for getting the best performance from the egpu and they actually had me uninstall the driver for the laptop.
you might be able to disable the GPUs you don’t want to use in device manager.