Additional 5K monitor for iMac27 2017

The user manual on the S90PC page shows a "S27C90" (same monitor?) as having the following ports (left to right, looking at the back of the monitor):

• One Thunderbolt 4 upstream port, marked with an illustration of a computer and a lightning bolt.
• Three USB-C downstream ports, marked with a variation of the USB symbol (horizontal "trident"?)
• One DisplayPort
• One 20 volt DC input port

I believe you're supposed to use either the Thunderbolt 4 port or the DisplayPort for video input. If you tried to feed USB-C (DisplayPort) or USB-C (Thunderbolt) video into one of the downstream USB-C ports, I don't know what the monitor would do with it.

https://www.samsung.com/ie/support/model/LS27C902PAUXXU/#downloads

Built-in Display:27" 16:9 WidescreenNative Resolution:5120x2880

Details:This model has a 27" LED-backlit 16:9 widescreen TFT active matrix "Retina 5K" display with IPS technology, a native resolution of 5120x2880, a "wide color" DCI-P3 gamut, and brightness of 500 nits.

2nd Display Support:Dual/Mirroring*2nd Max. Resolution:3840x2160 x2

Details:*This model can simultaneously support the internal display at full native resolution and up to one 5120x2880 (5K) external display at 60 Hz with support for one billion colors; two 3840x2160 (4K UHD) external displays at 60 Hz with support for one billion colors; or two 4096x2304 (4K) external displays at 60 Hz with support for millions of colors. It supports both extended desktop (second workspace) and video mirroring (internal display duplicated on the external display) modes.

Video (Monitor): 2 (Thunderbolt 3/USB-C) Details: This model has two Thunderbolt 3 ports.

These two ports support DisplayPort, Thunderbolt (up to 40 Gbps), and USB 3.1 Generation 2 (up to 10 Gbps). Adapters also are available to use Thunderbolt 2, HDMI, DVI, and VGA.

So, which port are you using on the Monitor?

And which setings on the Display, smoothing, adaptive etc,or whayever they're called on yours?

If you go to that Samsung support page, and filter by "OS: Mac" (on the left), it offers downloads for a Firmware Update User Manual and an Upgrade File (USB type).

I did not download and unpack the Upgrade File to see if there were any release notes describing exactly which monitors the firmware was for, and what it fixes, but you might want to look into that.