Since the LG is the wrong size in Displays, I think you need to make macOS forget the display:
In the following, you could change the files’ extension
.plist-old if you’d rather not delete them. You’ll need a password for the files in /Library/Preferences, and all of the files might not be present.
Everything should be back to the defaults, and you can set them up anew.
I just tried this procedure on my iMac Pro, and it reset the settings for my external monitor.