Hi,
You can use VS Code for developing apps and make tool to compile this app for your device.
The latest firmware for FX30 3G is Release 14
The firmware and source code are here: https://source.sierrawireless.com/resources/airlink/software_downloads/fx30-firmware/fx30-firmware-3g/#sthash.vcooPRen.dpbs
Please download source code and install toolchain, after that you can develop apps.
Below is some instructions for FX30
https://forum.legato.io/t/building-16-10-1-m3-for-fx30-r14/3555
Starting out with the FX30S and questions abound
All SDK versions on leaf are only for WP85xx, not FX30, so you can not use them.
Thanks,