USB is asymmetric. From the hub's point of view, "upstream" is the connection to the PC (the USB host), while "downstream" refers to connections to the devices.
The manual of the DELL S2718D says on p. 9
1x USB Type C (Alternate mode with DP1.2, USB
3.1 upstream port, Power Delivery PD up to 45W)
So it's not only a USB-C type port, it also needs alternate mode, so the USB host adapter in the computer you plug it into must be capable of this mode (and not all host adapters are).
Now it might be possible that the USB controller in the monitor is really really smart, and also supports an USB 2.0 connection, without power delivery or display port signals, where it just acts as a simple USB hub. In that case, an USB 2.0 to USB-C adapter will work.
But there's nothing in the manual that indicates it will do that, so the only way to find out is to either ask Dell, or try it out.
Edit
The manual of the LG 27UD88 says on p. 17:
The USB port on the product functions as a USB hub.
Please connect the USB C-C or USB C-A cable to the PC.
So yes, for data only, it can use USB 2.0. But to display an image, you'll also need a PC capable of USB C alternate mode (even though this isn't explicitely mentioned in the LG manual, but that's the only way DisplayPort can be transported via USB C).
Best Answer
On a monitor like that with USB ports, there is a USB HUB in the monitor itself. a USB hub has to have an input from the computer first.
http://en.wikipedia.org/wiki/USB
The TypeB the squared up connection is the input for the hub, a cable similar to a printer cable with a Type A plug on one side, and a Type B plug on the other side will be used to connect the computers USB port, to the hub. The hub will then have multiple ports.
Once you have an input run from the computer, the "HUB" will provide the USB ports. It will draw its power from the computer connection. Any USB connecting device would work in it, as long as it stays within the power specs.