So I spent the morning swapping around cellular modems, motherboards, and SIM cards. The bottom line is that the original X86J4105 and EM06-A I’ve been struggling with for weeks are simply defective when it comes to the cellular modem’s proper functioning with the SIM card and motherboard. I was surprised when I put the new known-good EM06-A on the original X86J4105, and it didn’t work properly. It was just like the original one. The next surprise was when I put the original EM06-A on the new X86J4105, and it didn’t work either. WHAT!
Only the new X86J4105 and EM06-A work properly and completely. This is all with my trusty old T-Mobile SIM card. This includes Windows 10 Pro and Ubuntu 20.04. It also includes not installing the Windows USB drivers because that really messes it up.