I’ve had a similar problem on 4.18 with blender 2.79. After about a week messing with export settings and googling solutions i’ve solved my problem. First of all, you won’t need to apply the edge split modifier, and connect back any split edges by using Remove Doubles if you have them. Then, mark all edges that are required to be sharp as “sharp” using Mark Sharp command or a similar button under the shading\UV’s tab. You can select edges by angle by using Select Sharp command as well as just marking them manually. Then, make sure your model does not contain n-gons by using Triangulate with an optional Tris to Quads command. Finally, in the export settings, under the Geometries tab, uncheck the Apply Modifiers option, set smoothing to Edge and check Tangent Space option. That’s the way it worked for me, at least.
PS. i’m sorry for any spelling mistakes i’ve made, and hope that the answer is still readable and helpful