If you have an active membership and used up the roaming data allowance then buying a new membership is not going to top the roaming data up.
You need to buy a data add-on as far as I know.
If necessary contact 48 support for advice.
See the link in my signature below or email them at support@48months.zendesk.com
While you're here, check out our "Featured Topics" below.
Our Troubleshooting Guide and Knowledgebase contain help and support articles and popular questions to help you as a 48 customer.