@lauraverigusche Please take a look at this link: https://community.o2.co.uk/t5/How-to-Guides/International-Roaming-and-Whats-it-all-about/ba-p/309678
If you've never used roaming before, you may have to speak to customer service to have it activated on your phone: https://community.o2.co.uk/t5/Welcome-News/How-to-find-help-amp-contact-O2-A-Guide/m-p/1201416
If you cannot call using any of the numbers provided in the link, you can call for free on any phone on +44800 032 1402.
Strange that it works for a day then just stops. Can you try a manual network search to see if you can select a different network to the one automatically selected.
You could also try rebooting your phone as sometimes that fixes lots of issues.
Do you have an update on your roaming issue @lauraverigusche?
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