Overlap only happens for volumes that can overlap.
I think the easiest way to build this, is to put a detector box right next to the wall, and show the widget when the player overlaps with the detector box, and remove it when the player overlap stops. (There are callbacks both for actor overlap begin, and end.)
In general, in Unreal, there are three main kinds of “collision”: The “bump into something” kind, which is called “blocking,” and the “overlap something” kind, which is called “overlap,” and the “cast a ray in the world” kind, which is called “tracing.” (there’s actually more to it, but this is a good start.)
Blocking and overlapping is automatic based on the physics simulation engine; traces you have to fire on your own explicitly.