I looked at it as well and @iDATUS is correct. I would also suggest moving the H2 above with an opaque bg so the line doesn’t go through it. I played around with it for a bit and it is not hard to accomplish.
I guess my main question is why do you have the timeline running through your entire page? You could use something like I did on this page: Company Information (scroll down to the production roadmap section) You will see that it changes for mobile but still has the desired effect. I only put lines and interactions within the section.
If you still want to use your way, all you need to do is set the “Section-Timeline” z-value to “Auto”. Then, go into the above section “Section BG Primary 2” and set that z-value to 5.