Recently, I came across a scenario wherein an ATT unlocked new Android device was procured and the device had to be tested. I had access to T-Mobile SIM. I thought of using that SIM and testing the phone. I was able to make and receive calls on the device. But, the mobile data access was not working. I tried to use WiFi, that was also working fine. After trying few other settings and restarts, I tried google search for T-Mobile data plan. One of the links I came across, resolved the problem. The link of about latest T-Mobile APN settings. In certain cases, the device automatically picked up the new telecom provide network and worked out of box. In case the Out of box doesn't work, it doesn't hurt to try the APN settings. This was useful to me. Hope it helps other as well.
