I have 2 monitors, exactly the same ( HP 1920/1080) the primary is running from a HDMI and the second a VGA. The monitor with the VGA doesn't recognise this correct resolution and I cannot find anywhere to add a custom resolution. The list of resolutions in the AMD properties window do not match my monitor on this list).
Which model of AMD hardware are you running?
If you don't know, then in Radeon Settings go to System -> Hardware and provide the Device ID and Revision ID.