Texting ROAMING to 150 is the method to enable international roaming - which is what allows a phone to connect to foreign networks. If this has not been done and the phone is now outside the UK, then you need to contact CS to resolve this.
If roaming is enabled, then lack of connectivity may be a local issue - toggling flight mode and manually selecting an alternative network often resolves this.
For the USA specifically, see also https://ee.co.uk/help/help-new/roaming-and-international/using-your-device-abroad/roaming-in-the-usa
