The issue is that the neural net framework of 11.3 was compiled against CUDA Toolkit 9.0, which is incompatible with the new Turing generation GPUs (ie the 2080). CUDA 10 was only released around 10 days ago, and it added support ("CUDA 10 is the first version of CUDA to support the new NVIDIA Turing architecture":
https://devblogs.nvidia.com/cuda-10-features-revealed
Neural nets in Mathematica 12 will use CUDA 10 and be compatible with your GPU.
