External Display recognized as DisplayPort, but connected via adapter using HDMI
The system is reporting that your Dell monitor is connected via DisplayPort be because you're actually connected to the DisplayPort port.  That said, it's not what's causing the problem you're describing.

What you're doing is converting the signal from DisplayPort (DP) to HDMI and most likely using a passive adapter; what you need is an active adapter or better still, just go from DisplayPort to DisplayPort (that monitor supports it).

I describe this in further detail in these posts:  


External monitors show blank screen until restart
MacBook Pro detects syncmaster (monitor) but doesn&#39;t display
USB-C to Displayport cable or adapter