Well with calls that don't go over wifi on that phone model
Low signal would cause dropped calls and internet to be slow like your complaints.
I would suspect that it is a low signal problem.
With wifi it does have a limited range. You would only get a good wifi signal if you are in the same house as the wifi source.
Sounds like need to have your cell coverage checked.
I am so sorry you are having these issues. theartiszan is definitely correct to check coverage. We can do that by contacting our online chat
Thanks so much