RE: Dell U2417H displayport daisy chain mirrors, but does not extend.
I do not think that the monitors are at fault here. Then the only other option is to test them on another computer. Preferably a computer with an added video card that has DP out or mDP out. Also contact HP support and ask if they successfully tested that system in a two monitor DP 1.2 daisychain.
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Run the PC pre-Windows diagnostics
Where do I find my Dell PC Service Tag or Express service code?
Out of Warranty Support