It looks like the OS ignores all changes I made in config.txt. For example, I rotated the display for my external touchscreen, I changed screen resolution in the config.txt. But it doesn't take any effect, why ??
On 1.3 it still works as expected.
In 18.04, they use tvservice program or raspi-config, I'm going to provide a frontend to tvservice in StellarMate Tools app in the next release to make it easier to change. For now, use raspi-config to select resolution.