To use Cellular Data, simply enable Data and Data Roaming on your handset.
Normally, no further set-up is required, however on some handsets it may be necessary to enter the Access Point Name (APN) manually.
The APN needs to be configured as follows:
APN: epc.t-mobile.com or fast.t-mobile.com
Username and Password can be left blank.
Below are the shortcuts to the APN settings for some popular handset types:
iPhone / iPad
Go to Settings > General > Cellular > turn ON Data Roaming > scroll down to Cellular Data Network > APN
Go to Settings > More > Mobile Networks > Access Point Names > New APN