Who is your cellular carrier? I have noted over years of commuting from Bellingham to BC that Verizon and AT&T handle the shift to Canada, and Canadian networks differently. This is important to the carrier as they get tons of complaints and demands for refunds when US customers inadvertently get connected to the foreign carrier, with huge roaming charges.
Verizon seems to use the location capability of the phone to cut off service when you cross the border, with a clear text message telling you what the consequences of connecting to the foreign network are. Service is reliable right up to the border line.
AT&T seems to have a broad, ill defined zone on both sides of the border where they block service. It seems like after crossing the border they keep your phone connected to their network, but deny service. Eventually the phone connects to the Canadian network, but the transition is unstable and ill defined. Similar effect returning to US, when southbound on I5 I get no service for 5 miles or so, when I had full signal northbound in the same locations.
Sucia and Turn Point are in very awkward locations for AT&T, both surrounded by Canadian cell towers on 3/4 of the horizon. My AT&T phone won’t work reliably at Sucia, my wife’s Verizon is rock solid.
If you are an AT&T customer, borrowing a Verizon phone to test might be a first step before a signal booster!