S2421HS, WD19S dock, DP not working?

I recently purchased a Dell S2421HS monitor to be used as a new second monitor for two Dell Laptops. I use a Dell Latitude 5400 laptop for work and a Dell Precision 3560 laptop for myself. When I use one of my laptops, they are connected to the monitors using a Dell WD19S docking station with the mounted USB-C cable. The first monitor is a Dell S2422HZ and is connected to the docking station with the provided USB-C cable. My current second monitor is an old 2004 Dell E173FP, which is connected to the docking station, using a DP to VGA adapter.

Now for the problem. The Dell S2421S monitor was provided with a HDMI cable which works when connected directly to one of my laptops or connected to the docking station. It is my understanding that one can not connect two monitors to this docking station using the USB-C port and the HMDI port at the same time. As a result, I need to use a DP to DP cable for the second monitor. When I connect the Dell S2421S to the docking station using a DP to DP cable, the S2421S does not work. I confirmed the DP to DP cable is working by connecting the Dell S2422HZ monitor to the docking station with this cable instead of the provided USB-C cable, where I tested both DP. 

I spent several hours on the phone with Dell Technical Support and we think it is a problem with the Dell S2421HS monitor. A replacement Dell S2421HS monitor is on the way. I am concerned the problem I am having is a design issue with the Dell S2421HS monitor and not defect with this specific monitor.

I look forward to your response.