In order to get the APN settings changed with an AT&T based SIM with Tracfone or Straight Talk this will explain what is needed. The phone was working but Mobile Data was not available. With that certain vendor SIMs in the Prime 2 there is no way to change or add APN settings - they are grayed out. I recalled I had a Textnow SIM via Sprint (totally free phone service and texting if you are in a Sprint area) which I put into the Prime 2 (no Sprint service is available at my location) but I was able to add the correct APN settings in the watch while on that SIM while it was in the Prime 2.
I then removed the SIM card while the watch was on (not especially recommended but it worked) and put in the Tracfone SIM which then showed the ATT nxtgenphone AND the Straight Talk (tracfone) APN settings which I can now edit.
I even put in another SIM which previously I could not edit and could not get Mobile data on and it already had the new APN settings. So apparently a SIM card is allowed to give APN settings to the phone which it keeps once they are set.