Welcome to the community!
I believe Fido is supposed to protect customers living near borders from accidental roaming charges (see here). You might consider contacting customer service to see if that particular cellular tower has been identified as being problematic. Alternatively, you might consider sending @FidoSolutions a PM. They should also be able to flag the cellular tower.
Your phone is connecting to the US cellular towers likely because their signal could be stronger than a Canadian tower. If you wish to prevent your phone from connecting to other cellular towers, you would need to manually select the Fido networks. You should note that manually selecting the Fido networks will also likely prevent your phone from accessing extended coverage. That is, your phone won't be able to connect to the other Canadian cellular towers (as Fido-EXT) when you are outside of Fido/Rogers coverage.
Hope this helps 😀