I have fiddled with the FBX export option, including setting Path Mode to “Copy” and choose to embed textures in the FBX file, but it simply shows the solid colour of the exported objects.
Although when deleting the “MixRGB” from the Shader Editor, it works fine, but then the texture’s background is in the wrong colour.
BUMP
I’m still looking for a solution to this. If anybody has a clue as to why the textures does not export with the MixRGB node, then please, do not hesitate to reply! 
Procedural nodes (i.e. pretty much any node that’s not an image texture with UV coords) are not supported by FBX… your “export” material should just be Image Textures plugged into the Principled Shader. See here for details about what node setup is supported for FBX export.
You can “bake” the procedural textures with a cycles emission bake. This tutorial from UGuruz explains how to “bake” your procedural textures for Unity (The principles are the same for FBX.):
Good luck
Thanks for your valuable input! Now though the scaling in the “mapping” node is not taken into account. Like so.
EDIT: I realise I need to have an image texture only, linked directly to the material. Although this is what it looks like in FBX Review when exported.
Doesn’t seem like the FBX exporter honors the mapping node… so baking is still your best bet.
Problem solved! Even though it took five days to get the uv-mapping correct, it’s done now 
