Incorrectly signed up for wrong plan. Requested a 20gb plan, was set up with a 4gb plan in error 

To avoid a complaint, customer services gave me a free add on. A further 15gb /month for 12 months, making a total of 19gb/month

Have been utilising my 19gb each month, this month after using 4gb, my data has stopped, and im getting the "youve run out of data, buy some more" message. More than two weeks before I get my next 4gb (and hopefully my 15gb)

Why would my add on... not be added on? 

Having to dive into cafes and shops to blag the WiFi, so that i dont miss an important email or message. 

Help!