Restart your device and check for signal bars. Turn off Wi-Fi to make sure you're using T-Mobile network. Set the device Network mode to Auto.Apple-users only: Reset network settings (tap Settings > General > Reset > Reset Network Settings) and verify the "T-Mobile Carrier Settings" are up-to-date. You can do this over Wi-Fi.Update the device software to the latest version. Perform a manual network selection to look for networks your device can connect to.