Solved by TheCADWhisperer. Go to Solution.
@TheCADWhisperer wrote:
Is there a trick to selecting a hidden edge for a fillet?
Click on the surface and hold untl you see a dialogue that let's you choose the edge that's hidden from view.
I couldn't reproduce what you describe, but in the process of trying it occured to me to look at the active Selection Filters.
This solved my problem.
That works too. I forgot to mention that you need to be clicking close to where the edge is. Heres a quick video to demonstrate.