Challenges that I see with that are:
- Purism isn’t going to make such a module for the Librem 5. You would be wanting some manufacturer to make a module. (To some extent this is the purpose of standards!)
- In order to use such a module, you would have to sacrifice one or other of the existing modules (cellular or WiFi) unless a vendor makes a module that does HaLow and one or other of the existing functions. 900 MHz is a cellular frequency so maybe there is an option to piggyback LaLow on a future cellular modem card.
- The module would need to be in the M.2 form factor and using (keyed for) USB, not PCIe or SATA.