Have a look at this guide here.

Hello ​@Federicamarchisotti 

Thank you for getting in touch with us here on the community. 

We are sorry to hear the service is not working for you. 

Is it just data that is impacted or are all services unusable?

Is roaming active on your handset and did you receive a text when you arrived in Turkey?

Thanks, 

Nat 

Hi. Thank s for reaching. 
i was in Turkey for the week end and needed internet connection on Saturday so i activated the add ons for the day for 5£. ( i only needed connection for maps ecc). All my roaming options even on the Id app were on. But it never works and the 5 £ will be added to my next bills which is unfair as i couldn’ t use the data. 
there is something you can do? 

Same problem here trying to use a 10gb addon in Mexico last week. Nothing I tried would allow me to use data. I’d like a refund - but there’s no way to speak to someone?!

Hi ​@MatWebb and ​@Federicamarchisotti,

If you haven’t been able to use the add-on and there is no usage we can look at crediting the account.

I would advise that you contact our Live Chat Team on https://www.idmobile.co.uk/live-chat.

Kash