you need to use OSD to switch the video input source from the AIO onboard gpu to the external HDMI source. Right next to the power button at bottom edge of monitor there is a small button called BIST (build in self test) When you press it it may show video input select source on screen, which allows you to choose external video in.  see #2 BIST  Solved: Inspiron 24-5490 AIO, how to use HDMI in as a monitor - Dell Community
I am trying to connect my new N-Go Spectrum Next clone to my Inspiron 5490 AIO using the HDMI port on the back panel. However, when I switch it on I only get a black screen on my AIO. Is there a manual way to switch over to the HDMI in port? I have tested the N-Go on my TV and it works fine.  
Inspiron 5940 AIO HDMI in port not working