Hi @carlrowan，thanks for your reply.
Yes, I tried to print the “decoded_data”, and it’s none. I use your RP2040 with rak4200 tutorial, which is the same code thatt you provided here: Raspberry Pi Pico Lorawan connection using RAK4200/RAK4270 Wisduo Breakout Board
Do I need to connect a sensor with RP2040 and RAK4200, to send data? Sorry in advance, if my questions are too basic, I am really new to this.
For your second suggestion, I changed APPEUI as 0000000000000001, unfortunately still couldn’t sent the “at+join” command when I use the RAK Serial Port tool. So I couldn’t move forward to connect the device to the TTN v3 as well.