I activated the crystal, but the portal isn't working. I jump into it, fall down and die. While falling down I can see the second part of the portal quite far from me. How can it be fixed?

have you used 2 crystals? there are 2 portal stands, one on either side of the gap. one crystal should be in the tower part itself, while the other is on the outside of the tower, accessible only by a scaffold to the left of the tower opening