- enable HiDpi if required
- install RDM latest release
- set custom resolution using RDM
SwitchResX -> create a custom resolution for INTERNAL display. Make it 2x the resolution you want, i.e. 3840x2160 for 1080p. Save it and reboot.
Then, connect your laptop up to your monitor. Select the SwitchResX icon in the menu bar at the top. Activate the custom HIDPI res for your internal monitor.