Same issue here, tried about 5 different things including this, but nothing has worked

Bump

I tried opening ("importing") the robot FBX file in blender, removed ("unlinked") the cube that surrounded the robot, exported as gLTF, opened in Reality Converter (downloadable mac app), exported it as USDZ and imported it to XCode. Everything works, only thing is that that the robot needs a 90 degree rotation. Next step is to edit the robot mesh into other characters.. We'll see how that goes :)

It's working now (October 2020)