If you can get one display to work from the dock, then it means the USB-C port of the laptop that the dock is connected to supports DP Alt Mode. You can also verify this using a USB-C to DisplayPort (CAC-1567) or HDMI (CAC-1504, CAC-2504, CAC-1586, etc.) or DVI (CAC-1510) or VGA (CAC-1502) adapter and connect a display.
The reason you can’t get two displays to work from the dock may be because the displays have a resolution/refresh rate that is too high? What exact Dell displays are you trying to connect?
It appears that the MST hub in the dock only supports DisplayPort 1.2 (HBR2 link rate) and only has two lanes. Two lanes is normal for USB-C docks that support USB 3.x. Your laptop only supports DisplayPort 1.2 so DisplayPort 1.4 support would not help.
You should be able to connect two displays up to 1080p60 using the MST mode. The total pixel clock for both displays should not exceed ≈360 MHz (for 8 bpc color). 1080p60 HDMI is 148.5 MHz but that can be reduced to 133 MHz using CVT-RB2 timing (if the display supports it). 1440p60 is at least 234 MHz so there’s little chance of getting 1440p60 & 1080p60 unless you lower the refresh rate (Intel GPUs don’t support lower bpc).
You can get USB-C docks that have MST hubs with 4 input lanes but those docks only support USB 2.0.