Follow up to this...
I went to the ATT store in person, rather than get a new phone number, a new eSIM was created on the 15 Pro Max phone. It still did not work.
Transferred the eSIM to the old 12 Pro Max (which had been factory reset in preparation to trade in) and the internet on Cell Data appeared to work.
Transferred the eSIM back to the new 15 Pro Max and the internet on Cell Data did not work.
The assumption was, something no longer installed or configured on the factory reset phone cleared out the problem but when returning back to the 15 Pro Max (which was transferred previously) this transfer between devices may have brought the corruption with it.
It was suggested once again to work with the Apple folks for additional troubleshooting (with the possibility of maybe identifying possible app problems or other diagnostics on the 15 Pro Max.
Diagnostics on the new phone passed without any apparent problems (the previous diagnostics on the 12 Pro Max were not there on the new phone so at a minimum some of the previously diagnostic problems were no longer a problems - i.e., the battery and/or wireless recharging did not appear to have problems any more).
It was suggested to Factory reset the new phone
After factory reset (without any apps installed yet), the phone internet over Cell Data worked.
There is no way to "selectively restore" files so it was necessary to go to the App Store, select the profile, and Purchases apps, and reinstall apps of interested.
Start reloading apps (not based on a iCloud backup) to help isolate if there was a specific app which might have been causing the problem or the corruption.
I used this as an opportunity to reduce the apps on my phone and focus on the ones I use more commonly.
Because the "iCloud backup" was not used at the time, many of my customizations were lost (i.e., my apps in folders - having to reorganized apps [unable to do so on PC either as iTunes app management not possible that I could tell at the moment], my preferences, and some app data which while may have been backed up to iCloud where not able to be restored per an "iCloud Restore" or "app data stored in the cloud"). As one example, one of my password management apps was unable to be restored fully so I lost some of by passwords.
As of this moment the phone appears to still be working but was unable to identify a specific app that may have caused it.
Given some of this started when moving between locations, I'm curious if some sort of "roaming" activity could have in someway corrupted the SIMS, network, or connectivity and just got stuck that one. But I'm not sure.