You have to use the portal that is almost directly below you (slightly to the right) to go over to the Rift side and move a stone out of the way.
If you want to see what the Rift side looks for this area, this link takes you to @Roeland 's maps for this event,
Scroll past the video at the top of the page. There is a scroll-able set of 7 thumbnails. Click on the 5th thumbnail to open the page for the area you are working on. Look at the bottom left for the portal marked 20. A yellow arrow points to the stone you need to move.