First power on the AIO without connecting hdmi cable to laptop.  Is there an “on screen display” (OSD) to let you switch from pc to monitor only?  Manual does not seem to provide clear guide how to do it.  If you don’t find it, you need to install OSD driver first.  Here is OSD driver for 5400 AIO. This package contains the application for the Dell On-Screen Display (OSD). The Dell On-Screen Display (OSD) allows you to control the brightness bar, change the input source, and turn the screen on or off through the OSD button. PS a year ago another user found the following was a solution to similar problem in 24-5490. “There is an HDMI-in port on the back and the manual says, "Connect a gaming console, Blu-ray player, or other HDMI-out enabled devices.". However, when I plug my another laptop into it, my laptop actually can detect 'Digital television Inspiron', but I cannot switch the AIO display from its own windows to the laptop's. So how can I do that?”  That user found installing OSD driver was the solution.the driver for 5490 OSD.  
Inspiron 5400 AIO does not work as an external monitor (HDMI-OUT to   HDMI-IN). The laptop detects an external monitor, but the monoblock's screen is black.
inspiron 5400 AIO - extending screen to dual monitor - problems