[Q] Cannot set APN (T-Mobile US) topic

I inserted my microsim card into the machiine and connected to Wifi. However it did not automatically download carrier's mobile network setting as a lot webpages described. So I tried to input APN settings manually. But as soon as I input everything and hit save, it went back to the original APN setting window without showing the newly added APN EVEN IF I selected Save.

Has anyone else had this problem?