Yes, provided you're connecting the same WhatsApp account and phone number, and you haven't reset or cleared data in the WhatsApp app, all the messages should still be there.

However I don't know anything about transferring WhatsApp data to an iPhone, as I've only used WhatsApp on Android devices.

 

Portability across platforms has been said to be coming for a long time, but I haven't heard that it's fully there yet. A couple of years back you could find workarounds on the Web but I don't know how well they work.

Just switching the SIM back and re-activating WhatsApp on the android should fix the immediate problem.

 

I've swapped my SIM back and forth between old and new phones before now, and just logged into WhatsApp each time.

I don't understand what you are saying about re-activating the phone and providing you with a SIM though. If you are still with ATT surely you can just put the SIM in the old phone: that is the whole point of a SIM card (I've never in 30 years "activated" a phone with a service provider, just the card, and I don't tell them when I change phones). Or do ATT actually check the IMEI and not allow customers to use their network unless you've asked their permission to use that handset (technically possible, but most networks worldwide regard that as a waste of effort and money)? Of course if you've changed carriers and the android only accepts ATT SIM's that will be different, but in that case ATT won't be able to give you a SIM with the same phone number (as that number would now be associated with your new account). So I'd be tempted to just try the current SIM in the old phone myself.

 

Apparently there are various ways to do that, some of which described here.
https://www.imobie.com/support/how-to-restore-whatsapp-backup-from-android-to-iphone.htm

 

Suggest you look at the URL I posted previously about how to transfer WhatsApp from Android to iPhone. Of course I can't provide any more help than that, as the last iPhone I had was 13 years ago.