I have a few business apps that won't work with root so am currently trying to get the stock Nexus 5X RSAP working in my 2016 Audi A7 but I'm seeing the exact issue you describe in your "SIMFix" thread:
"The current Audi car phones have problems with some SIM cards. Sometimes the connection starts and aborts after a few seconds, sometimes the connection works for a full ignition cycle but fails the next time"
I know about the bug where the bluetooth won't disconnect on the Nexus 5X (I'm working around by auto cycling BT with tasker) but I'm also seeing the above intermittently. I was wondering if you could go into a little more detail about what causes the issue above? Would the same problem affect the stock RSAP? Is there a workaround or do I need a new SIM card?