Enter Advanced config mode:
Enter Multi-center mechanism:
Change Max reconnect interval vaule to : troy
Save config,reboot DTU.
Troy mode SIM Light and STATUS Light flash alternately.
Now you can sand AT command through COM 1.
Exit at mode. After entering at mode, the serial port is always connected to the module, so it is unable to log in DTU directly with configuration tool to read / download configuration and other operations. If you want to connect DTU with configuration tool, you need to connect to COM2; or connect PC with serial port, press ESC key of keyboard / send exit with serial port tool.