If you're buying so many roaming add-on's, have you considered subscribing to a Canada/US plan? They start at $34/month for 50GB and includes calls and text.
For roaming to work it's best to set your phone to 4G preferred vs 5G, because AT&T 5G doesn't work (but it may connect to it anyway). And if automatic network selection doesn't connect you to T-Mobile or AT&T, you'll need to manually connect to one of them. I've never had issues with automatic network selection, but I've heard some iPhone customers say their phone tried to connect to Verizon, which won't work as they be are not a roaming partner.
As for roaming add-on's, you can't immediately buy the same add-on back to back. I.e. once the 15 days lapses, you'll need to wait until midnight (or 24 hours?) before the system will let you purchase the same add-on again. Alternatively you could log a ticket and ask an agent to do it earlier for you.
