How can I create a theatre marquee animation using paths and dash lines?

I would like to create a looping animation that has dots moving along a rectangular path with rounded corners.  I have seen a tutorial showing how to do this with an open path that has 90 degree angles. The continuous path was drawn with the Pen tool and then made into a dash line. The line was converted to a symbol that was given a number of frames (10). A Shape Tween was applied. A keyframe was created at the end frame. Then the starting point of the keyframed line was shifted by a distance equal to the length of the dash plus the space.  It works pretty well.  But when I tried to do it with a rounded corner rectangle or a series of horizontal and vertical straight lines joined by rounded corners the animation only affetec the first line.  It's as if they were a series of disconnected separate lines.  I don't know how to make a single continuous path. There is no "Join" function in Animate as in Illustrator.  I any case I would appreciate suggestions on how to create the desired marquee effect whther by using dash lines or some other - hopefully non-laborious - method.