After quite a long and difficult troubleshooting process with a bunch of trial and error, our Asia/Singapore servers are back online under the same IP address.

It is still not clear what the cause was but we narrowed it down to the server's A2S query response's UDP packets getting blocked when trying to reply to the game client's second A2S query (with the challenge number appended).

This could be due to a router or OS bug, but we now know how to more quickly identify and reconfigure should this bug happens again.

Thanks for your patience!