One single ticket, KMG-BKK-SIN?
Any checked bags?
Both flights on TG metal?
How long is the connection?
Generally speaking, you should be able to check your bags through to SIN, and utilize the mid-field int'l-int'l connection, BP check, up one level, clear security, go to lounge/gate.
The exact details of your itinerary might yield different situations, e.g. no BP issued in KMG for BKK-SIN, so you'd have to queue at the connection counter.