You might have a data service for your phone, but many phone network operators block people from using their phones as a modem unless they pay extra.

Call your network operator's support line, explain that you want to use your phone as a modem for a computer (which is what the tablet is technically), and ask them if it's included in your phone account plan. If it's not included, they may ask you to pay extra, and that will unlock the modem service as well as the data service.