With the “command successful” response from the modem, I’d bet the modem is good to go now for VoLTE, and the issue is with something else. But the issue could be with a different setting, or with AT&T. Again, this is just a hunch, but I’d say it’s worth a last tinker with the easier to change settings. Mine is still working with PureTalk, on AT&T backbone, but maybe they are less restrictive with the phones on the MVNOs.
So the settings I’d look at are under the Settings -> Mobile. On mine the following works:
Mobile Data On
Data Roaming Off
Network Mode 4G Only
Network Pure Talk [of course yours will be AT&T or something]
Access Point: RESELLER (under the name Pure talk, again, yours will be att or something here)
SIM Lock Disabled
Modem Details is just informative, but I’m curious if yours says “lte” under network type.
In the end, I’m glad you took the leap and gave it a shot. I know it’s tough to convince yourself it’s a good idea.