Since you're really close to the US there and the US towers are putting out stronger signals than Canadian ones, it's not uncommon to switch to roaming if you're quite close to the border or a US tower. Happens to me whenever I'm in White Rock BC too. Turning off roaming is the main option. If your phone isn't connecting to any of the Canadian towers when close to the US border, you may want to contact tech support directly when you are on a Canadian network.
Telus has one tower on the south edge of Sarnia at the Suncor refinery. The rest of the towers you'd be using are from Bell and they cover the whole city. That includes three along the river. (Cell tower map)
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