Use in the USA

You'll need to purchase an out-of-plan add-on to use your SMARTY plan in the USA Faye.

In addition, to use mobile data in the USA you'll to setup the APN as detailed in the online help article:

https://help.smarty.co.uk/en/articles/2045756-setting-up-your-phone-to-work-abroad 

When I looked at roaming costs for the USA, tears came to my eyes, and these were not tears of joy. 
Anyway, I decided to purchase a data only eSIM to use for mobile data in the USA, at a reasonable price. You can always just stick to free Wi-Fi networks, and WhatsApp or something similar.