The information for the 4052 H transponder on Apstar 6C looks to be up to date on lyngat, several of the channels you were first looking at and receiving (CTN Cambodia) changed last year to HD on that transponder, but one channel still is still in SD.
The HD channels are now in MPEG-4
DVS-2, the new symbol (SR) rate is now 10400, the FEC is still 3/4, there is one channel still in SD (CNC) it is still in MPEG-2, however the channels RF modulation has changed to 8PSK, this may be your problem.
I don't see in your receiver specifications that it can receive 8PSK type modulation, all it can do is QPSK, unless there has been a hardware change, or an add on card for the higher channel modulation your stuck.
Do you have a spec sheet for that receiver? As the one I looked at may be out of date, it shows all it can do is MPEG-2, QPSK.
This is all I can come up with as far as a manual for that receiver.
Strong SRT 4300 Manual Online: Specification. 1. Tuner & Demodulator Input Frequency Range RF Input Signal Level RF Impedance RF Bandwidth LNB Power LNB Tone Switch DiSEqC Contro Demodulation Symbol Rate 2.. MPEG Transport Stream Input Rate Video Audio Aspect Ratio Video Resolution...
www.manualslib.com