Uwe Brauer composed on 2017-08-02 08:18 (UTC):
If Trinity Control Center's peripherals display size & orientation screen's
It did not but xrandr returned someting.
select list of settings for screen doesn't allow other than screen 1, then install arandr from Ubuntu's standard repos and try it.
Thanks for pointing that out, meanwhile I obtained the relevant xrandr command for my setting.
Uwe Brauer