This works for my Dell Ultra Wide monitor.
defaults write -g CGFontRenderingFontSmoothingDisabled -bool NO
defaults -currentHost write -globalDomain AppleFontSmoothing -int 2
Credit to Chris Hoffman: https://www.howtogeek.com/358596/how-to-fix-blurry-fonts-on-macos-mojave-with-subpixel-antialiasing/