You indicated it was a day trip, not an overnight trip. One thing that might have resolved this problem was to have started your Roam Mobility plans 24 hours before your return time. If you're not sure exactly what time of day you're returning, start the plan 24 hours before the latest return time you could imagine it stretching to. Instead of having most of the excess time at the end of the trip after your return, this way you'll have it at the beginning of the trip before you arrive so that delays in the plan activating may be resolved before you hit the border.
You're right that network problems can't be resolved by Roam, but the more time you give them to get your SIM activated the more likely it will be active by the time you need it. You're already paying for that excess time, you might as well be strategic about it. I usually fly to the U.S. so I set my plan to end slightly after the plane is scheduled to leave on the final leg to Canada, which means it's generally active early in the morning (or even the night before) that I leave on my trip.
Lumia 950 XL dual-SIM -- SIM slot 1: Rogers (small business flex data) -- SIM slot 2: Public Mobile ($120/12GB/90 day plan)
Bookmarks