Hello Petre,

sorry for this late response - hopefully it is still a bit helpful though! I would rather treat these graphics seperately. Furthermore please notice that I added a certain thickness to the tube. Obviously the conversion to a BoundaryMeshRegion does the trick:

ClearAll["Global`*"];
SetDirectory[NotebookDirectory[]];

cube = Cuboid[{10, 10, 0}, {20, 20, 10}];
cubeBoundaryMesh = BoundaryDiscretizeRegion[cube]

Export["cube.stl", cubeBoundaryMesh]

tube = SolidData[Entity["Solid", "CylindricalShell"], "Region"][2.5, 2.4, 5];
tubeBoundaryMesh = BoundaryDiscretizeRegion[tube, MaxCellMeasure -> {"Length" -> .1}]

Export["tube.stl", tubeBoundaryMesh]

Regards -- Henrik