[X Style (Pure) Original Android Development] kb8no:
I have 5 working Clarks. One is for my wife on 17.1 clean flashed from Stock on Verizon SIM and works and I do not touch it. One is on 14.1 because it is stable, everything works (VoLTE etc) and I use it to stream music. The other 3 I just play with and I have a Verizon and an ATT SIM for testing. I clean flashed one from 17.1 to 18.1 (2/20 update) using LOS Recovery. It seemed good. Then I placed the Verizon SIM and as I described, it would start in LTE and drop back to 3G and "DM Config Update" error shortly after boot. Then it started to deteriorate and became laggy and erratic to the point of being unusable. So I began to wonder if I should try to start over with stock with the SIM in place.

I wondered if the SIM was corrupted as well so I put it in my "old faithful" 14.1 phone. It worked as it should and fixed the SIM problem. I then put the SIM back in my play phone and started over. I tried it - flashed stock and booted it and and SIM worked fine. I then did a clean flash of 18.1 with LOS Recovery. It started up perfectly other than same "DM Config Update" error. LTE started up and stayed working. ROM worked perfectly and not erratic, like a different phone.

I then decided to try my Verizon SIM in one of my other play phones on 18.1 and same problem, LTE started up and quit and then dropped down to 3G. So I put the SIM back in the previously working perfect phone and, guess what? SIM was corrupted and it would not stick on LTE any longer.

So I thought either I was going crazy or I had discovered something weird. So I started all over. Put the newly corrupted Verizon SIM into my 14.1 phone and it worked. Then I repeated the same process with the 18.1 phone, put in the SIM, flashed and booted stock (worked), and clean flashed 18.1 with LOS Recovery. LTE worked perfectly again, did not drop down to 3G, and works like a bomb.

Then I have a play phone on 17.1 using my ATT SIM with no problem. I tried that SIM in a 18.1 phone. It came up as "ATT" but no data. I put it back in the 17.1 and it was corrupted, same problem and it did not work. I put it in the "Old Faithful" 14.1 and still corrupted. Geez. I gotta go get a new ATT SIM now. I gotta believe if I take a phone back to stock with a working ATT SIM and then clean flash 18.1 it will work but I am worried.

So something is not right with the way the SIM initiates on 18.1. If you want 18.1 to work with a SIM I suggest you flash back to stock and bring it up to 18.1 with the SIM in it. Then don't let your SIM's cohabitate with other phones. I also felt the phone worked dramatically better after flashing back to stock. It might be good just to flash back to stock and clean flash to 18.1 on general principles.

Addendum: Well Well Well. My ATT SIM was "suspended" because it was not a "supported" phone on 18.1. I can't say that 17.1 worked either because I haven't tried it for months and it stopped working immediately after I tested it first. VoLTE doesn't work on 17.1 either. All the providers are doing away with OS's that do not support LTE or VoLTE .

The ATT SIM was an easy fix for the technician to "restore", just like starting a new SIM card. Now I am guessing that the problem with the Verizon SIM may have been a variation on the same problem but rather than suspending it they dumbed it down from LTE to 3G rather than suspending it. Flashing the SIM back to stock and immediately to 18.1 may have just been a workaround.