I've made my research but nothing seems to work.
I tried everything that the post below and many others said. My issue is exactly the same.
My problem is that for some reason Blender only exports the action I have selected when I export it to FBX for Unity.
I have exported FBX models with animations to Unity and everything was fine, however, other models have this issue.
I made sure there are no red lines for missing bones, tried exporting with many different combinations of options, applied scale... I don't really know what else to do.