MIUI trouble with AT&T - the APN mismatch

How to use AT&T data in a phone with MIUI
Bugs over bugs and out of date informations - that’s when ATT meet MIUI

Story

Re-entering the US after 2 years and late for the start of the semester, I desperately need a stabe phone number for all the registrations. I have a list of about 10 items waiting for a number to continue.
So I went to the local att store and the madam seems new to my phone but still confidently set me up. After leaving the store, I found the network is not working. The signal is full, but not network IO exists. I can call and text but not data. After rebooted for several times, I realize there should be a systemetic error in either my phone or att. It’s probabaly gonna be the APN problem.
I go back to the store 40min before my next meeting. The dealer took 30min asking his/her colleague for help. They finally believe my APN statment when the 3rd guy came. He showed me a web page of APN settings and taught the other two what should they do next time. Up to this time, I do not have enough time. So I went for the meeting and took a photo of the APN settings.
When I tried to set APN myself, I found it never works. I was always told that I am not allowed to edit the APN by the system even though the interface is designed as it’s for editing. The new APN I typed in with effort just disappears after I tap “save”.
Searching with google and baidu, I found many people has the same problem but no one really solves it. The best solution from the internet can make the network works but I have to turn of 4G and 5G, type in the APN, watch APN disappear, turn 4G everytime after rebooting the phone.
Back to my phone, I found only changing the first setting will not make APN disappear. So I screened for which setting term cause the bug. It was the MNC. The default is 280 but the clerk told me to enter 410. And I found this link. The clerk is telling me the wrong MNC! That’s why it cause the bug! Is is 280 on my ticket but no one in the store know how to read that.

Solution

The key is not to change the MNC.

• Go to 双卡和移动网络 and click the ATT card
• Go to the APN settings
  • 名称：NXTGENPHONE
  • APN：NXTGENPHONE
  • no change
  • no change
  • no change
  • no change
  • no change
  • MMSC: http://mmsc.mobile.att.net
  • 彩信代理: proxy.mobile.att.net
  • 彩信端口: 80
  • MCC: 310
  • MNC: [keep default! Change this number will cause the bug. The default number should be the same as the 4-6 digit of the SIM number on the ticket]
  • not change for the rest
• tap 更多 on the bottom and save