ACARS does not equal transponder so what you see on FR24 is not ACARS data.
and
What frequency (or band) does the ADS-B transponder transmit on? Or is it sent to a satelllite?
ACARS transmits around 135MHz (VHF). The actual frequency depends on the region of the world where the plane is. At these types of frequency, once can expect reception to extend to perhaps 250nm.
ACARS signals are sent from the a/c and received by a ground station, for onward processing etc.
AFAIK, the relevant ground station in that region is Hat Yai in southern Thailand.
Here is a copy of SITA's map of their VHF ACARS ground station coverage in Asia:
Although the map resolution is not so good, VHF coverage exists throughout the region between KL and the Vietnamese coast. The only area where no coverage is shown is a region to the north of Phuket Island (west coast of Thailand) and out towards the Andaman & Nicobar Islands, (but there does seem to be an ACARS ground station somewhere on the latter islands).
HF ACARS is also used, with Hat Yai again the relevant ground station, operating on 5,655MHz and 13,309MHz.
ADS-B operates on 1090MHz (UHF). Transmission is generally over a line of sight path.
From the above, you can deduce that when a plane is outside the range of reception for VHF ACARS or UHF ADS-B, no data/signal can be received, unless an alternative channel (satellite or HF is used).