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In "Device Manager" under "Monitors" does it still show the old Monitor? If it does, under "Properties" uninstall it (be sure to click "uninstall from disk"). Also if you don't see the old Monitor, In Device Manager under "View" click on "Reveal Hidden Devices" and see if your old monitor is there. If it is uninstall it.
Since both are LG monitors, Windows may still be seeing the old LG driver installed on the computer.
Under Windows Settings - Display, can you change the display to something below your new monitor resolution? Also on the same Settings as Display at the bottom click on "Display Adapter Properties" under "Monitor" be sure your HZ is correct for the new monitor.
What type of HDMI-D-SUB adapter are you using? the best and most compatible adapters are the "Active" type rather than the "Passive" type unless you are using a HDMI-VGA cable.