Hello Xianlong198x,
Welcome to the community!
Do you live close to a border? Occasionally, phones close to the border might connect to a US cellular tower rather than a Canadian tower. Many of the affected areas have already had this issue addressed. However, there may be a US cellular tower which has not yet been identified to Fido as problematic. Fido is supposed to have an accidental roaming solution in place for those who live near the US border (see here and here). It's possible that particular US cellular tower has not yet been identified as an issue. You would need to discuss the matter with customer service so they might identify that location as an issue. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
If your phone still connects to the AT&T tower, you might consider manually setting your phone's network to Fido. Doing so should prevent your phone from connecting to the US cellular towers. However, you should note that it would also prevent the phone from accessing extended coverage (Fido-EXT), if required.
Hope this helps 😀
Cheers
