My reading of the situation is that it's a terraced house with no rear access.And secure the property as a matter of urgency.
You say that "joe" may be accessing the property but you can't.
That doesn't make sense to me.
But it's very difficult to follow your posts.
It's a bit like reading Joyce or Beckett.
The front door is boarded up so the OP can't get in that way
The back of the house is only accessible from outside via a neighbour's garden (over the wall as described), so there's no access for the OP that way.
The only way for the OP to make the house secure is to open the boarded up front door, secure the back of the house from inside, then board up the front of the house again.