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?
Chances are that the actual issue had nothing to do with the software version but was merely something else that coincidentally resolved itself at the same time as you switched versions.
But why, practically, does the old version matter to you when you have the new one which works?
When there’s concern about software changes, the usual first place to look is the release notes, and then escalating to the commit history of the source repos.
Also, understanding failure is a debugging process - “not possible to join” is only an initial observation, understanding where the join process fails is what matters.
