Now I have more strangeness to report. I no longer have cellular internet in Windows 10. I made the mistake of installing the Windows USB drivers for the EM06-A. That was a big mistake. It enabled the various USB serial ports and gave the cellular modem its real name, and killed the connection to T-Mobile. Uninstalling the drivers didn’t fix it, though it did put back the cellular modem name and removed the serials ports as I expected.
I then installed a new NVMe SSD and installed Ubuntu 20.04. I now have cellular internet, but only in Ubuntu. Ubuntu has all the drivers for the USB serial ports as well as the EM06-A cellular modem, unlike Windowss 10. This says that the new hardware is good.
I will need to investigate the original hardware to see if I can get it working.
Right now I’d say the Windows USB driver for the EM06-A breaks the cellular internet access.