I have been struggling with this for over week now too. I initially bought S2B modules for my senior project, to use over UART with a Raspberry Pi.
Somewhere between design and construction, the S2B modules became obsolete, wish DigiKey would have told me, and the rest of my team bought S2C modules. I can confirm that the S2B modules work perfectly with my UART Pi hat, but the S2C modules will only work with a USB adapter, so it sounds like we are in the same boat.
Even though Digi claims these modules OTA compatible, the system does not like the UART and I do not want to redesign my Pi hat for SPI, which the new modules also have.
So, I ordered a new set of modules from Digikey and started playing around. I played with all the configuration settings. I even downgraded the firmware from 4059 to 4055.
I did eventually get the modules reliably communicating using the UART, but I spent hours (a lot of hours). I exported my configuration, so that I could give it to my classmates, but it does not work for them. It seems that while I was experimenting with the modules, I had recovered them to "XB24C reg" hardware instead of "XB24C reg TH" hardware.
So, I ordered yet another set of modules from DigiKey and still cannot get them to work. I am using the settings from my now "XB24C reg" modules, but no success. The modules will link, I get an RSSI led, but I cannot get UART data across my Pi, so I think there is something wrong with the firmware that is preventing us from using the modules.
I hope someone will look into this further.....