I suspect you've missed the point i made earlier that certain features are not available because they aren't in the firmware unfortunately. You can set your location to a rural area to see if DEM [Digital Elevation Modelling aka 3D Terrain] is available to it. Try different zoom levels. When you zoom out beyond the detail map you'll see another entirely different representation type of 'DEM' which is built into the OEM Basemap and is more like the colour shading of a printed paper map.
There are two complely different Garmin features which confusingly both have '3D' in the naming, 3D Terrain [DEM] and 3D Buildings & Landmarks. A unit's firmware might have the capability to see the 'fake 3D' of Buildings & Landmarks [like a moving 2D representation of perspective drawings on paper, but it's not really 3D] however that unit might not be capable of displaying DEM [3D Terrain], an example of that is the nuvi 7x5 series. However nuvi 3xxx series can handle both in the firmware. I know the 2595 can't even use the 'fake 3D' of the gmap3D.img [they only show as 2D outlines which are probably still only polygons build into the detail map anyway which it can display regardless] so maybe it can't process the specific DEM map images in it's firmware totally regardless of what's [apparently] enabled in region 140/supported_features.html.
Again, hard reset is the Immediate Action/First Aid to ensure all features are available following any manipulation of the unit's firmware or regions.
If any map type is unchecked in Settings>Map & Vehicle>MyMaps it will not be available to display regardless of what's checked in Map Layers and vice versa. Can't have one without the other.