The permission WRITE_SECURE_SETTINGS is required to use this app. To grant it, run this command:
adb shell pm grant com.draco.smresolutionchanger android.permission.WRITE_SECURE_SETTINGS
I will not be held responsible if you crash your device by using an unsupported resolution or density.
I recommend taking a screenshot in case you need to enter your default settings, and the default button isn't perfect.