iPhone 13 pro doesn't work on Bell Canada mobility 3G networks

3G is going away. Most carriers are in the process of shutting down or have already shut down 3G service, so you can expect performance to suffer if 3G is still operating in your area.

Check with your carrier for details on their plans. They are likely shutting things down by year end.

https://www.zdnet.com/home-and-office/networking/3g-is-shutting-down-here-are-the-gadgets-that-still-rely-on-it-do-you-have-one/

Bell Canada is shutting down all 3G service on December 31, however, they have already started shutting down locations selectively, so there may be no nearby 3G towers where you are, which means your connections will be weak reaching far away sites.

What did Bell Canada say when you reported the problem to them?

The final sunset date for 3G in Canada isn't until 2025, but that doesn't mean they're not starting to phase it out now.