Which MATLAB version are you using? Since R2021b, the importONNXNetwork and importONNXLayers functions automatically generate custom layers for ONNX operators that they can't convert to built-in MATLAB layers (in many cases). So that might help you with importing. You can also use importONNXLayers, and import the layer as a placeholder layer, which you have to find and replace with a custom layer to keep the settings you want.
And finally, you can get a pretrained U-Net MATLAB network without having to import. Go to MATLAB Deep Learning Model Hub.
