Black border around external display.

I have a 2021 14-inch MacBook Pro (M1 Pro chip). I am connecting it to a Prism+ W280 Ultra display - a 4k 28-inch display. I am using the HDMI cable that came with the display to connect to it (HDMI to HDMI). I am using it as an extended display at a resolution of 3840 x 2160 (4k).

For some reason there is a black border around the display (where there should be pixels). How can I get the picture to cover the entire display?

TIA.

Here are screenshots of the setup: