does a sim card from an iPhone SE 1st generation work in an unlocked iPhone 12 mini or does it require a new sim card?

Contact AT&T. I’m with AT&T, you need to get one of their “unlimited data plans” that includes 5G. I swapped Nano SIM card from my old iPhone 6, into my new iPhone Pro 12 Pro Max with no issues.

I know post is solved. But I should have wrote that I have 5G service from AT&T, and I never need to get new Nano Sim card. I just got one of AT&T unlimited data plan with 5G, and 5G appeared in my iPhone 12 Pro Max.👍

That was my same experience as well. I didn't really explain it that well in my post either. I just had to swap my plan out from my current unlimited to their 5G Elite plan. Funny thing is that given that I have 5 phones and a Watch, just making that change is actually cheaper than my current unlimited, even with the same discounts I had with work, DirecTV, auto-pay and all that. I also received a text that their spam relief was to be included at no cost, I just needed to activate it.

The SIM from the iPhone SE should work in the iPhone 12 Mini, however if the carrier requires a special SIM to use for 5G, you would need to get a new one from the carrier.

I did not require a new one when I got my iPhone 12 Pro, but you can check with them if you wish. A quick call can verify it.

No worries. I switched to 5G Unlimited Starter plan from sort of tier plan. 5G plan seems way cheaper, probaly prorated, or something. Don't know until next month.🙏