not dimensionally stable

Unfortunately, the cake ring retains not the set size, but shrinks namely jerky again.
So you have to resort to a trick and fix the ring.
This works best with a clothespin that one clings to the upper edge, after having set the desired size. Of course, the staple for hygienic reasons should be unused and if possible made of plastic, so you can wash it off.
Once you do that, the ring can be used well. If one does not, and relies on the words of the manufacturer, to the cake mass was then released in the refrigerator ........ very freely.