Hi,
I hope my experience could help someone who may have the same needs as me.
After so many trial and errors, I finally found out the reason is that the default kernel doesn’t support RFCOMM. In order to enable RFCOMM, we have to recompile the kernel. Here is a great tutorial for recompiling the kernel (NVIDIA Jetson Nano - Docker optimized Linux Kernel · Docker Pirates ARMed with explosive stuff). Just follow it, then in the menuconfig panel, enable the “RFCOMM TTY support”, which is under the “Networking support/Bluetooth subsystem support”.
Then enjoy!
Best regard,
Xiaoqun Yu