second display compatibility with Samsung

What year model iMac?

What model is the Samsung Display?

1) Not all cables are created equal, some will work and some won't. Even the Samsung supplied HDMI cable may need to be replaced with a premium certified HDMI cable.

2) Apple suggests using the USB-C to AV MultiPort adapter or the Belkin USB-C to HDMI adapter for an HDMI equipped Display.

For more information and other connection options,

see > https://support.apple.com/guide/imac/connect-a-display-apd8cdd74f57/mac

I was referring to using,

the > https://www.apple.com/shop/product/MUF82AM/A/usb-c-digital-av-multiport-adapter

or the > https://www.apple.com/shop/product/HKQ22ZM/A/belkin-usb-c-to-hdmi-adapter

that Apple offers and a premium certified HDMI cable.

It looks like the other option that you have with that Samsung Display, is a USB-C to DisplayPort cable.

for some examples, see > usb-c to displayport cable

again the Samsung and/or cheap-o cables may or not work.