@Shanook79When roaming a number of factors kick in such as the local network, which o2 have no control over.
The first thing i would do is try and connect to an alternative network and test that it works from that
Also check your apn is set to mobile.o2.co.uk
Also it might be worth contacting o2 to see if you have any roaming bars on your account.
To contact Customer Services dial 202 from your o2 phone or 0344 809 0202 from any other phone
Current Phone: Sony XPeria Pro-I and Motorola Razr 5G
I also have a Planet Computers Gemini
I work in IT as an Enterprise Messaging and Collaboration Tools Administrator (Office 365, Slack and much much more) and manage hardware both Mobile and Desktop / laptop, my desk is a bit like PC World
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We are all customers and dont have access to any o2 accounts