Try locking the cat in the room where the box is - especially at night. When you see it using the box for several days, try to gradually extend the space where the cat is allowed (ex. Add another room to her initial territory), until she learns the new environment, and you see that it continues to use the box. Continue to extend her territory until it is free to walk through the whole house, but still goes to the box room when it needs to. Even if it is a street cat, she should be trainable. She just needs to get used to the box before she gets used to the whole house. Let me know if this works. :)

Is that your kitty in the photo? It is BEAUTIFUL!!!

Best regards!

 Not all pets are house trainable and if he was trained before I would question the training since it appears that this is an outside cat.