For 3D printing, you can NOT use material definitions like height/bump/displacement maps!
a 3D print model is like a clay model. Everything must be modeled by vertices and faces.
If you are using modifiers, like subdivision,. You need to APPLY them for making a PRINTER .stl version.
So what you see is what you get. Blender smoothing doesn’t work for 3D printing.
Low poly means flat faces.
Apply subdivision to a low poly model and apply the modifier. The higher the subdivision, how smoother the 3D printed version will be. Depending on 3D printer setting and resolution.
Blender has a 3D printer plugin, see preferences. It can help you in checking the model.