About Gateway RAK2245 Version and Chirpstak Server

I was using LoRa Device RAK5205 and gateway RAK2245 (S/W version 4.2.5R). However, it was not possible to join the Chirpstack Network Server.

I changed RAK2245 S/W version 4.2.5R to RAK2245 S/W version 4.2.7R, and it was resolved.

Why can’t I use Chirpstack in RAK2245 S/W version 4.2.5R?