When two players are close to each other, they are moved in to a new Network, which is a new temporary voice channel. However, in my server, the discord client fails to connect to that voice channel, showing RTC Conneting... and No Route alternatingly.

If I right-click the voice channel and change the server region from Automatic, which is default from the discordsrv bot, to Japan, for example, it works from then. (BTW I am in South Korea)

Have a look at the screen recording I made:

When the new channel was created and the players were moved into that, the connection isn't established. You can see the error message. At around 0:23, I change the server region from Automatic to Japan, and it starts to get connected.

I think if you provide an option to set server region overrides in voice.yml for temporary voice channels, the problem would be solved. Discord's official API already supports this.

I have to do a major event on our server in a couple of days with 30 people. I hope this issue resolved soon.

Not possible currently discord-jda/JDA#1570