I have been to Montreal with my Koodo phone and had excellent reception with my iPhone 5s but had a different phone one time and it wasn't as good. Could possibly be your phone is old or needs an update or it could be you just hit one of very endear spots which happens. I live in maitland ont. and get pretty perfect service but if I go into brockville I only get two bars on LTE in places. Weird but it happens.
I live in Oakville and my Rogers service sucks but it's not just me. All my neighbours suffer as well. When I owned a i6+ the service was OK as I think the phone was really good at pulling in a signal.
Some areas are just not good. My wife's i6\5s and previous i5 meanwhile get great Telus/Bell service.