Welcome to the community!
Do your parents have cellular services with Fido/Rogers as well? If they do, do they also have the same issue?
Cellular services are dependent, in part, on the location relative to the surrounding cellular towers. You can get an idea of where the cellular towers are located here. If you've always had this issue when visiting your parents, it's possible they live outside of Fido/Rogers coverage. On the other hand, if the issue is new, it's possible there has been increased interference (ie new building, etc) between their location and the cellular tower(s).
What phone do you have? Does your phone have a setting to enable roaming (not data roaming)? Most newer phones do not have a separate roaming setting. If your phone does not have a separate roaming setting, then it should automatically roam on the other partner cellular towers as Fido-EXT (extended coverage). Since your phone can only make emergency calls, it suggests that your parents might also live outside of the coverage from the other cellular towers.
Hope this helps 😀