@Katayoon
you can log in to your Public Mobile account when you get to USA, using wifi at airport, local library, most cafes have free wifi now...you might have to ask for the password. Once logged in you can buy the desired add-on at that time. The add-on will have a 15 day time limit. If you need it longer than that, might be better to upgrade to a Cdn/US plan.
For talk, you MUST have a cell that has VoLTE recognised by Public Mobile as not all cells are. Without that you won't be successful making or receiving calls while in USA. Here's some reading with a device checker at bottom of the article...
https://www.publicmobile.ca/en/get-help/articles/volte
