@Gingerbeard You do not need add ons, you do need to enable data roaming in the phone settings, and if 5G doesn't work try 4G/LTE on any of AT&T, T-Mobile and Verizon. For voice calls your phone needs to be VoLTE capable and enabled since the USA has shut down their 3G networks. There's a list of VoLTE "certified" phones here, others may or may not work. If your phone is not on the list there is a test you can do while still in Canada so let us know.
https://www.publicmobile.ca/en/on/get-help/articles/volte
