I have a cellular-capable iPad Air running iOS 8.1. When T-mobile had their 200mb per month for free promotion a while back, I jumped at the opportunity and got one of their SIM cards and activated the promotion. I have been using this data since then when I am on the go. My problem is that Messages/iMessage has never worked while using T-mobile cellular data. I can neither send nor receive messages, but when I am on Wi-fi data, it is fine. Other apps that use data work fine. Any advice on how to fix this?