After messing with it for a good while, I was actually able to get it working with 5G/4G on the T-Mobile network by flashing the Android 13 stock image, then using the UBports installer. Note: it did seem to have some issues entering the fastboot mode, so that took some finagling.

Also, it didn't seem to like it the first install, and I think I flashed Android 15 before Android 13, THEN installed Ubuntu Touch. But mileage may vary, obviously.

Now to run some documentation of all loaded kernel modules and firmware versions from dmesg, and potentially try to debug why sound doesn't work on PostMarketOS, enough eyes, every bug is shallow, etc, as I'd prefer Plasma Mobile over Loromi. (Lorimi?) [But any FOSS system over Android, so I'm happier than I'd be without Ubuntu Touch]