Details: We purchased an iPhone 12 pro max 256GB from Apple store in US, unlocked, model # A2342 on the first day that it became available in the store (11/13/20).

The phone number we had was on Simple Mobile and we inserted our sim card in the new iPhone 12 and it worked out of the box, including the data.

About a month ago, we ported our phone number to AT&T and activated a prepaid service on it (unlimited data $65 plan which is $50 with autopay).

The next day we received an AT&T prepaid 5G sim card in the mail and our troubles started from then, while voice and text have been working, the data has never worked with the AT&T service on our particular iPhone 12 pro max. We have perfect reception in our area.

We have contacted AT&T daily and worked through their debugging for couple of dozens of hours so far and their back-office is trying to help through an escalated ticket, but not a slight amount of help or improvement has happened so far.

We have done some experiments ourselves and this is the summary:

- the new AT&T prepaid sim card is placed inside an AT&T locked iPhone SE2 and it works out of the box, the data is at 5G-E, but it works fine and it is very fast too.

- the new AT&T prepaid sim card is placed inside an unlocked Samsung S8 and it works out of the box.

- a post-paid AT&T sim card is placed inside the iPhone 12 pro max, same issue, voice and text working, but no data at all.

- a pre-paid Tello sim card (which uses T-Mobile network) is placed inside the iPhone 12 pro max, data works fine, it shows LTE, since the service is not 5G.

- a freemium FreedomPop/RedPocket (not sure if service is on AT&T or T-Mobile) is placed inside the iPhone 12 pro max, data works fine, it even shows 5G.

- a pre-paid Page Plus (on the Verizon network) is placed inside the iPhone 12 pro max, the service is not recognized after couple of minutes.

- during this past month, on couple of dozens of times, we have reset the network setting of iPhone 12 pro max, turned it off and back on, and put it in airplane mode and back out of it, turned off the Wi-Fi, and in none of the times we could get cellular data through AT&T prepaid or postpaid sim cards. Also when cellular data is ON (and not working) it shows always 4G besides the antenna bars, despite that in the settings we have it set to 5G always ON.

- today, finally we backed up the iPhone 12 pro max to iCloud did a full erase / factory reset on it. Tried to set it up and even during the early set up process, before it asks for Apple ID, we asked it to use the Cellular data to restore from iCloud and didn't set up any Wi-Fi connection for it. It was showing 5G besides the bars and started to read from the iCloud, obviously somewhere along the way of restoring the iPhone, it would get the stored Wi-Fi password and connected to it. While we were excited to see the cellular data working after this, nothing happened at the end, still the same issue exists.

- AT&T has not been able to help us for almost a full month now.

- Apple store where we had bought the iPhone 2.5 years ago, had the two suggestions we already tried, put the pre-paid AT&T sim card in another phone, and factory resetting the iPhone 12 pro max which we did today, and it didn't help.

We appreciate any insight and experience that you can help us with.