There has to be an APN setting, or the phone can't connect to the internet. You may not be able to edit the settings, but try adding a new one, then setting it to use the new one.
From TMobile's support page:
"Tap Settings > Networks > Tethering & networks > Mobile networks > Access Point Names. If available, tap the T-Mobile US APN (the bullet point fills with green). If not available, tap the Menu key, and then tap New APN. To reset your APN settings, tap the Menu key and then tap Reset to default"