Mac mini 2012 with Dell U2717D & Dell U2417H
There are two things I would try at the outset: Resetting your SMC and NVRAM. Instructions follow.

Resetting the SMC on your Mac Mini

There are a number of ways you can reset the SMC on the Mac mini. My preferred option on older Mac minis is to follow these steps:


Shut down your computer
Unplug it from power
Press and hold the power button for 5 seconds
Plug it back in and turn it on.


Note: If you prefer, you can also use Apple's latest steps for doing this instead. If so, ensure you're using the option for Reset the SMC on Mac desktop computers.

Resetting the NVRAM on your Mac Mini


Shut down your machine. Yes, a full shut down, not just logging out.
Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
Hold those keys down until your Mac reboots again and you here the startup chime.
Let go of the keys and let your Mac reboot normally.


Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

Let us know how you go.