I installed the 17.2.1 update on my 12 Pro right after it was released before Christmas. Now, after more than three weeks while traveling to family during the holidays without a working cellular connection, and more than 20 hours on the phone with both Apple and T-Mobile Support, and a couple trips to Apple Stores, I finally have a working phone as of a couple days ago. However, the problem was never identified or fixed - they finally just gave me a new phone. I am writing this sad saga in hopes that it helps many of you who have the same problem.
After many hours in a half dozen sessions with Apple technical support downloading diagnostics on my phone, the person that I was dealing with said that it was clearly a software issue, and specifically that my phone hardware was fine. In fact, the cellular connection started working at one point while they were collecting diagnostics, and then stopped again for no apparent reason. Luckily this was all documented, because she then recommended that I take it to an Apple Store. They wiped my phone clean, but before restoring ran their own diagnostics, and said that it was a hardware issue, so refused to restore it. They then offered to repair the phone for about $600, basically the price of a refurbished 12 Pro. At this point my phone was a brick.
I refused to pay them because I knew that there was no problem with my phone until their update destroyed my ability to make a cellular connection. It was as if someone at Apple said, "Give me your phone, I'll make it better for you," and then hit it with a big hammer and handed it back. And when I complained that it wasn't working, they say that it was my problem, and that I'd have to pay to have them fix it.
I re-connected with my Apple Technical Support person, and told her that I was unwilling to wait for Apple to figure out their software problem - that I needed a working phone now. She spoke with a supervisor in Customer Service, to whom she connected me. That person asked me a bunch of questions, and after initially saying that it was a hardware problem, relented and authorized a replacement 12 Pro for me at no cost. I found one at a local Apple Store, went in, and traded in my non-working phone for the "new" one which is probably refurbished.
I then had to take the new phone to get a new sim card from T-Mobile, because they had told me to destroy my old one when my phone stopped working. T-Mobile had remotely installed several eSims on my old phone which had worked temporarily, but all eventually ended up not connecting to cellular.
After installing the new sim card, I went back to the Apple Store to have them watch while l restored the new phone in case there was a problem. I didn't want them claiming that I'd damaged the new hardware if it didn't work. The backup that I restored from was done after the 17.2.1 update, which they said shouldn't be a problem, but had me concerned.
It took a while but the restoration process worked fine. Now three days later my phone is still working and connecting to the cellular network. No explanation of why my other phone stopped working, or why this new one with the same 17.2.1 update is working. So the lesson is, don't let them tell you it's a hardware issue - insist that they replace your phone which they damaged with their update.
Makes me wonder if their software somehow causes a change in the phone that shows on diagnostics as hardware damage. And there is nothing from Apple explaining what happened, or even admitting that they caused this issue with so many of their users. Hopefully someday they come out with a new update that fixes the cellular connectivity issue, as well as the many other problems apparently caused by this update, which appears to have been rushed and not adequately tested.
Just don't pay Apple more money to fix the problem that they have caused. Assuming that there isn't a software fix soon, find someone at Apple who can get you a new phone to replace the one that no longer works because of their defective update.
