Thanks for taking the time to raise this via the forums, we'll do all we can to answer your query.
Before leaving the UK to go on your travels we'd recommend ensuring that the roaming service is fully set up with ourselves. This can be done via calling the team on the usual number 789 or 0345 600 0789 or via your online account.
Once abroad a reboot of the handset once you have landed is always a good idea to refresh the connection, you may also need to ensure roaming is available and set up on the actual handset which will need to be checked via the manufacturer.
If you struggle to get a connection, try changing from 'automatic network selection' to 'manual'. You may also need to try this in different locations as the connection is completely dependent on the local networks availability adn signal strength.