Hi @Anubhav , That's very odd that J8 is not working. It's a hard short to the 5V power subsystem that feeds the board. There is no limitation I can see that would prevent that.
I know for Spektrum based receivers, there is a specific 3.3V power sequence and bit-wiggling that occurs for binding (through Flight Core J12), so I'm not sure if your receiver is expecting something similar and does not like the "always on" 5V on M0018 J8 and J9.
The 4G LTE modem M0030 ports J5 and J6 are safe to use. They expose a 500mA limited 5V to those pins. They are USB2 ports @500ma, with USB HS data on pins 2/3, and DGND on Pin 4.
They are turned on sometime after power-on once the hub enumerates with Voxl, so I wonder if that delay ON time is what is helping your receiver bind and communicate properly.
You might be venturing into a Software domain which I am not an expert in, so if just knowing that J5 and J6 are "safe" and this works for you, then great!
If not, we may need to pull in some SW folks to dig deeper.
Thanks!
Vinny