Needs to be properly re-done. Get a contractor, remove the landing and see what's behind it and have it properly repaired, install flashing, and re-do the landing or the porch whatever you have there.
Anything else you try to do at this point will be a bandaid and it will only make things worst down the road... You already have moisture issues at the sill of the door and it's working its way from the inside out.
Good luck
Q