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Has the displayport cable ever worked?
If you have another DP cable, or device that outputs on DP it may be worth testing that.
It's probably an output rather than an input. My old Dell monitor (a U2414H) allowed daisy chaining via DisplayPort.So the issue is the monitors I'm using. They're Dell 24" with 2 X DP, 1 X Mini DP and 1 X HDMI. It turns out that the second DP doesn't work and seems to be a dummy port. In the menu there's no option for DP2 so originally I thought it auto detected but no, it's just not connected.
It's probably an output rather than an input. My old Dell monitor (a U2414H) allowed daisy chaining via DisplayPort.