Problem with extra screen

To my new Inspiron 3525 with Win 11 and its HDMI connector, I wished to connect my old monitor AOC e950Swn. However, the screen then says "Input not support". However, I have managed to overcome the screen's resistance to my computer by changing both "Desktop resolution" and "Active signal resolution" in the monitor settings to the resolution requested by the additional screen. So now I have a picture, but only when I select "Computer screen only".

The choices "Duplicate" or "Extend" certainly gives a picture of both, but incomplete. Furthermore, the pointer is on one but opens windows on the other. If I choose "Second monitor only", both screens go black and it is difficult to bring the computer back to life.

What should I do?

/hans peter