Ok, I got my sim card today and then started looking as to how to change the APN settings. First I found this thread (thanks to Google), then I found the solution.
The sim-swap deal seemed rather kludgy for an Apple product, and all I had was the dead AT&T sim that came with the phone so that was no help anyway.
Then I found this - the iPhone Configuration Utility. Right here on Apple's support site. (This link takes you to the KB article, follow the links to download)
It's a little daunting but after a little trial and error, I managed to create a config profile that works great.
First thing is after your StraightTalk sim is active and you can make and receive calls, check to see if your texting and MMS is already working. Mine was, I guess since it was an ATT phone in the first place. If it is, you don't have to mess with it, all you need is the cellular data settings. If not, good luck. (I can't help ya there)
Download and install the utility. They have Mac and Windows versions available
Run it then hook up your iPhone
Click on Configuration Profiles
Click on the New button
You'll see a whole list of possible settings. First click on General.
Give it a name... I called mine WebAccess
Give it a unique ID. it has to be "com.blablablawhateverwhatever.profile" (Obviously, replace everything in the middle with whatever you want)
Give it a description. (mine? "Hopefully Grants Web Access")
Now scroll all the way down to APN and click on it
Click the Configure button that appears in the window on the right
You'll notice right away that there aren't enough fields to include everything they say you should.
The MMSC, MMS Proxy and MMS Port settings weren't needed anyway since MMS was already working.
Access Point Name (APN) - att.mvno
Access Point User Name - leave blank
Access Point Password - leave blank
Proxy Server and Port - proxy.mvno.tracfone.com : 80
Click on the Export button
Click on the dropdown and select the "Create and sign encrypted configuration profile for each device" option, make sure your iphone is connected and highlighted and hit export. You'll see an install window pop up on your phone, hit the install button. After it loads it on your phone it lets you save it to your computer as well. Name it anything and save.
Restart your iPhone. It should be online and good to go. IF you get stuck like I did, the help menu in the program will take you to the knowledge base online help for the utility. Very well done, answered my questions and got me going. This all looks like a lot but really, it only took about 10-15 minutes and that's with a couple shots in the dark.
Good luck! I hope this helps someone out there.