Windows 10 thinks old monitor is still attached

I have a dual monitor setup. Originally, I had a Samsung 32" UJ59 monitor and a Dell 24" S2409W monitor. This configuration worked fine but I decided to upgrade to have matching 32" monitors.

After removing the Dell 24" and attaching the 2nd Samsung 32", the system still insists the 2nd monitor is the Dell 24" and absolutely refuses to recognize that the 2nd monitor is now the 32" Samsung. Consequently, I cannot set the monitor to it's native resolution of 3840x2160, I am not able to set the resolution higher than 1920x1080 which was the native resolution of the Dell 24" monitor.

I have tried uninstalling and reinstalling the both the display adapter and monitor from device manager. I've tried different cables and a different display adapter but still the machine thinks the 2nd monitor is the Dell. I uninstalled and reinstalled the display adapter driver as well as the monitor driver. I even tried to use the Radeon software to create a custom resolution but it refuses with a message saying that the resolution isn't compatible with the monitor (it also insists the 2nd monitor is the Dell).

Here's the specs for this machine:
ASRock B365 Pro4 Motherboard
Core i5 8600K @3.6Ghz
8GB RAM
Radeon 7750 GPU
Windows 10 Pro 64-bit (20H2)

Any suggestions for how I can get the machine to realize that the Dell 24" is no longer attached?

Did you check Device Manager and Monitor to remove the Dell then reboot to clear it from memory?

Thanks for the reply Berton. Yes, I had already done that.

I purposely left some info out of my post above which is that I am using a KVM switch to switch the dual monitors between 2 computers. I knew in the back of my head that the KVM switch was probably the issue but didn't want to deal with it and was hoping it was something else.

The fix was actually pretty simple, I just had to unplug the display from the KVM switch and plug it directly into the computer. This allowed the system to correctly id the monitor. Once I did that, then the monitor was fine when I plugged it back into the KVM.

Need to chalk this one up to OHS (Operator Head Space)!