Since RTH is only using the stored location that the Phantom recorded when you started it up, mobile tower/wifi issues won't affect it at all.
Not if you switch to dynamic home point, as far as I understand. I experimented with RTH, and the drone attempted to land 60-100 meters away from me and starting point in some cases in urban areas. Maybe I was just unlucky, but dynamic RTH can be affected by tower/wifi issues from what I can tell. What can possibly prevent that ? It doesn't look like DJI app is forcing satellite location, does it ?