Adding a billing add-on is just that, a billing change. The ability to connect to local networks abroad, i.e. the ability to roam, is a network setting not a phone one.
On the basis that you're on pay-monthly, did you enable international roaming before leaving the UK? If not, you will have to call EE C/S from another phone (+44 7953 966250) or via a web-calling app (+44 800 0798586) to do so.
With roaming enabled, your phone will be able to connect to foreign networks. Calls, texts & mobile data will then function.
