Does your phone show coverage indicators of a US network? From your description, I suspect it doesn't - and if so, your current deal has nothing to do with this.
What I suspect you've not done is enable international roaming before travelling abroad - this prevents your phone connecting to foreign networks and will cause the symptoms you describe. Did you text ROAMING to 150 before leaving the UK?
If not - you will need to contact EE CS from another working phone, or use a VoIP service such as Skype, to request international roaming is enabled. If you want to add a billing add-on such as the Roaming Pass, you can request that at the same time - but the lack of a billing add-on will just impact on your costs, not your ability to roam.
This advice all presumes you are on pay-monthly and not PAYG.
