VoLTE works on AT&T/GSMA, but with no menus and no visual indication (unless enabled in the Hidden menu) many people didn't realize it was there. Of course AT&T and most MVNOs didn't provide it either. And no VoWiFi.
The HD icon is a sign that you have VoLTE working, probably VoWiFI too since they use the same codecs. I don't get the HD icon on GSMA except when I tested H931 ROM.
You can look for TMO specific menus with Shortcut Master Lite and relevant settings through the Hidden menu. But make careful note of anything you change, as some of these settings persist even after Factory Reset and re-flash.
It sounds to me like it's all working, just not very visibly. I did indeed get the impression from other US998 users on TMO that they had more menu controls and more visibility. Again, I assume they must have been on Oreo.
It may be by design that Pie doesn't show icons: Since VoLTE/VoWiFi is becoming the only ways to make phone calls, I can see the logic of not putting multiple icons on the status bar to confirm that "this phone can make calls".
You can consider rolling back to Oreo, just realize there is a risk of messing things up. Definitely go via Nougat, and use Partition DL.
I am very busy these days and the next few weeks, so may not be able to respond as much to any questions
Edit: If you didn't perform a Network Settings Reset, I'd say definitely try that. Also, you could try re-loading the IMS configuration in Hidden menu. Restart the phone after both.