ethminer lists eGPU but doesn't use it

[mintho]

I'm new to this, so excuse my ignorance, but whatever I do, ethminer only seems to find my internal graphics card, not the external one.

mintho$ ethminer --list-devices

[OPENCL]:
Listing OpenCL devices.
FORMAT: [deviceID] deviceName
[0] Intel(R) Iris(TM) Graphics 550
	CL_DEVICE_TYPE: GPU
	CL_DEVICE_GLOBAL_MEM_SIZE: 1610612736
	CL_DEVICE_MAX_MEM_ALLOC_SIZE: 402653184
	CL_DEVICE_MAX_WORK_GROUP_SIZE: 256
[1] GeForce GTX 1080 Ti
	CL_DEVICE_TYPE: GPU
	CL_DEVICE_GLOBAL_MEM_SIZE: 11811160064
	CL_DEVICE_MAX_MEM_ALLOC_SIZE: 2952790016
	CL_DEVICE_MAX_WORK_GROUP_SIZE: 1024

mintho$ ethminer -G -F http://my_pool_address --disable-submit-hashrate --opencl-device 1

[OPENCL]:Found suitable OpenCL device [Intel(R) Iris(TM) Graphics 550] with 1610612736 bytes of GPU memory

As you can see, when ethminer lists the available devices, it does see my eGPU, but when I try to start it with that same device in the argument, it again only finds the integrated graphics chip.

I've read that setting opencl-platform to 1 helps, but in that case, it doesn't find any GPU at all:

mintho$ ethminer -G -F http://my_pool_address --disable-submit-hashrate --opencl-device 1 --opencl-platform 1

No GPU device with sufficient memory was found. Can't GPU mine. Remove the -G argument

Any help would be very welcome indeed.