Hi @nghiant2710
Your guide is exactly where I started. The problem is you can not specify a specific port for the GSM modem in the /resin-boot/system-connections/my-gprs-connection configuration file. This makes sense because Modem Manager auto detects the port on which a modem is connected.
As I describe in my original post, Modem Manager does not consider a Raspberry Pi’s UART as a valid port to scan during auto detection of modems. This is because the Pi’s SOC’s UART is blacklisted in the Modem Manager code.