Does your phone show signal indicators of a local network? If not, then your roaming facility is very likely inactive.
Adding a billing add-on doesn't enable the roaming facility which allows your phone to connect to foreign networks. Within your myEE online or app, you need to ensure the "international roaming" & "call abroad & premium rate" options are active.
If you can't enable these for whatever reason, you will need to contact EE C/S to do so. Either +44 7953 966250 from any other phone, or +44 800 0798586 from a webcalling app such as Skype.
