Voice and sms work fine. Android is up to date etc. Network connection is set to automatic and I got the welcome message:
Welcome! With iD, you can roam free in 50 destinations worldwide. Use your minutes, texts and data as you would at home (fair use policy and surcharges may apply). Keep in mind that out-of-plan rates could cost up to GBP10.24 per 100MB. For more info, visit idmobile.co.uk/roaming or dial +447482227777 free from your iD phone. Enjoy your trip!
I notice the ID info within the app for roaming mentions call and texts but not data. The message above does include data though and there's no Add On for International data. Hence everything suggests it should work but apps do not work and an Speedtest shows no data is received or sent.