Nvidia GeForce 940mx and dual monitors.

The Dell D3100 hub uses DisplayLink technology to create "indirect displays".  Essentially, displays attached to the D3100 hub aren't physically connected to any GPU at all.  Instead, a DisplayLink driver on the system uses the CPU and GPU to compress display data bound for the dock-attached displays, then transmits that display data as regular USB data traffic (which is how you can have displays attached via regular USB even though regular USB has no native video support), and then a DisplayLink chip inside the dock decompresses that video data and sends it out to the displays.  The upside to this solution is that it allows you to use docking stations with systems that don't have any "traditional" docking station connector and/or add more external displays to the system than you have "real" built-in display outputs.  But DisplayLink also introduces significant downsides.  First, the compression consumes system resources and can result in display artifacts, particularly when large display areas are changing at once, such as when watching full screen video.  And second, due to a Windows limitation, in a multi-GPU system, only the Intel GPU will be able to accelerate content on DisplayLink-attached displays, so even if you have a discrete GPU, you won't be able to use its horsepower for anything you're showing on those displays.

Looking at the specs of the Inspiron 7560, it looks like all you've got is an HDMI output -- no underside docking connector or USB-C/TB3 port.  In that case, you should use the HDMI port, even if it means a dual cable solution.  If that's already in use for something else, then you don't have a better option for getting multiple displays attached to that system.