Kerlink offers iFemtoCell that has opkg based system, provides toolchain, and an open firmware (though the firmware itself isn’t open sourced).
This may or may not be an interesting alternative for OP.
It is simple to use, though not turnkey as you are free to use any packet forwarder thus none is preinstalled. Fully compatible with TTN and easy to use (a couple of configuration files to edit in order to register TTN’s server and frequency plan).
Disclaimer: I work for Kerlink.