Dell monitor and IMac mini

Here's Dell's description of the SE2422H:

https://www.dell.com/en-us/shop/dell-24-monitor-se2422h/apd/210-ayxt/monitors-monitor-accessories

HDMI and VGA input ports, 1920 x 1080 resolution @ 75 Hz.

I wonder if the power cable isn't fully plugged into the monitor. Dell's picture of the monitor's power connector shows a socket with 5 prongs in it, one of which might be a bit recessed. It might be worth disconnecting the power cord from the wall outlet, gently trying to push the monitor end a bit further into the monitor socket (you don't want to force it, and break anything), then plugging the cord back into the wall and trying again.

What year model Mac mini?

What model is the Dell monitor?

What cable connection are you testing?

For more info on connecting a display to the Mac mini,

see > Connect a display to Mac mini - Apple Support

In addition to what Servant of Cats has suggested, try a better quality premium certified HDMI cable.