Take a look at T-Mobile Internet and MMS settings: Apple iPhone to make sure all of your APN settings are correct. If your settings are correct, take a look at Wi-Fi troubleshooting: Apple iPhone. There you will see information on how to troubleshoot issue while connected to the T-Mobile network, and wi-fi.
The answer supplied isn't helpful, the iPad Mini doesn't have MMS settings for one thing and this doesn't address whether or not T-mobile LTE spectrum is in the iPad Mini range. These of s that use iPad Mini's on aT-Mobile network have no idea how your towers are configured and no way to find out. So is it to be hit and miss for us or what? The iPad Mini isn't just a big version of the iPhone 5. Please advise. Thank you.