Hi @plaferriere - Welcome to the particle community! You’ll find a lot of great information here and lots of other particle users willing to help out.
I would strongly recommend starting with Rickkas7 tutorial on Github including examples. https://github.com/rickkas7/RF9X-RK
I started with his tutorial step by step and never ran into a flashing red SOS error. I original was using a Particle Boron + RFM95 radio feather only talking via LoRa to multiple Adafruit M0 RFM95 LoRa feathers. That said, I am now contemplating using all Boron + LoRa radios to maintain OTA capability in the remote LoRa nodes.
Regarding your SOS flashing red. I’d start with a minimized version of the example and keep building up from there to identify where the problem is. Honestly though, if you assemble/solder it per that GitHub tutorial you should just about be able to take the example, load it and be sending basic Hello messages between the two. If there is anything else you need guidance on feel free to post or message back and be sure to check out other forums here. For example, here is another decent one that was the last few weeks discussing long range IoT (i.e. LoRa and other methods). Long Range IoT Network
I’d be curious to hear more about your use case is.