That may come down to how Bell has the towers connected in the area. Telus has no towers near Tillsonburg and shares service on the Bell's towers. Have you contacted mobility support to advise them of the issues? If you haven't, I'd recommend doing so. You can either do that by phone or by clicking on the green Report an Issue button on the coverage map page.
Fibre coming to an area wouldn't affect cell service. The Bell tower next to Gyulveszi Park appears to have a generator connected to it on street view so it should be operational for a time when power is out. It may depend on how large of an area is without power and what infrastructure Bell has in the area.
It is a blackhole from Tillsonburg to Mt Elgin and again from Tillsonburg through Delhi. I have a business in Delhi and used to be able to call clients on my drive but I do not now as they always drop. I have had telus for 3 years and it has only been bad for the last year. My family has better luck with their Bell phones but not much. I am considering going back to Rogers for this reason.