As you can see in the image below taken directly from the U2415 page, the Apple proprietary hardware, operating system, and drivers were not tested/validated by Dell.
We at Dell are not trained in the usage of the Apple OS. You should ask the Apple users on their Forum for troubleshooting opinions.
I can tell you that the U2414H does not support the usage of USB-DisplayLink scenario. Ask the Apple users about USB dock options.
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