And I moved my ATT SIM, from my iPhone 11 Pro Max to my iPhone 12 Pro Max.
I purchased a SIM-free iPhone 12 Pro Max and used the SIM from my iPhone X, My plan is Unlimited Elite and 5G is working fine.
This is exactly true. The old SIM should work just like it did in your 11.
I did exactly the same thing. I passed on the option to purchase with an AT&T SIM - just felt 100% confident I would get a completely unlocked 12 Pro if I bought one with no SIM. I took the SIM card out of my previous 11 Pro and everything looked good (I do NOT have a 5G plan currently and it's at least a year away where I live). Then I started to realize that there IS a newer AT&T SIM card and as mine was at least 3 years old I went into an AT&T store and asked about a new SIM. They looked at me like I had two heads. About a week later I went back and this time they acknowledged there was a new SIM - but they didn't have it in stock and to call AT&T. I called AT&T and asked them and they said "Sure - we'll send one to you - you'll get it tomorrow". Tomorrow - and several more tomorrows came and went and I finally traced the order in my account and found out it was back ordered. I called them again to see if they could tell me when they expected stock and they told me they had no idea but try a store. So I went back and the store told me they'd be getting them in in a couple of days. So back I went in a couple of days - and they told me they were out but getting more in. So I called the store in a couple of days and was told they were out. But then said "Wait a minute" and put me on hold - in a couple of minutes I was told they checked with the manager and they DO have a few in stock, so I said "I'll be there in 15 minutes" - and I was. A simple process to put in the new SIM and off I went.
Question: I moved my SIM from an iPhone 7 Plus to an unlocked iPhone 12 Pro Max. Should I purchase a 5G SIM card and should I upgrade to a 5G phone plan?
Buy one, NO. If you need one they won't charge you. Upgrade your plan, MAYBE. Depends on where you live, what it's gonna cost you and what you think you'll need 5G for. In most areas of the US, except for some of the major population centers, 4G/LTE is just as good and just as fast.
I would answer this based on your service area. I live in NH and 5G is pretty much non-existent. I just swapped my old SIM and have my old plan. Everything is working as it should.
The 11 doesn't support 5G. The icon you're seeing is "5Ge", which "is AT&T's misleading name for an enhanced 4G LTE network [...] and is not actual 5G connectivity."
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