When you or your users need to use a Dell AIO as an external monitor? Well, I must say that Dell has quite a few model lines so owners and users have to check the source (or video connections must be configured and done in the right order to prevent a no signal display message.
Computer to Dell AIO
As you are utilizing Windows, you will need to use the screens mirror or extended feature to cast to the Dell AIO. The Dell AIO needs to have at least one video IN port (usually HDMI since it carries audio and video in one) but you may have a Dell AIO that only accept a VGA signal and has to run an additional cable for the internal speakers or utilize external ones.