On the side a 3d visualization cube is not possible so far I'm in 2018 sp4 version, I'm also looking to do it but it doesn't work, it's just a visual pity.
I did a few tests, (SW 2018 SP 5), at first, I was able to dimension the visualization cube on an iso view without any problem, but on the front and top views, impossible to select the lines of the visualization cube. I then did a right-click and "Restore sketch visibility" (nothing to see in my opinion) and there I was able to select the lines of the visualization cube. On the other hand, I did the same maneuver on another view, and it didn't work.
Conclusion: Dimensioning the visualization cube is not yet developed under SW!! To be tested in future versions for those who are lucky enough (or have the means) to be able to use it.
Sorry, I don't know where the problem can come from, I can mishandle the 3D and 2D of my example part, show or hide then redisplay the visualization cube, for me it works all the time without any problem.