This is physically fiber cable problem. Same as mine previously, where when it rain, LOS light would be blinking. Found out that there was a dent (very minor dent) at the main cable from the distribution box into my house. So need fix by laying a new fiber cable. After that my internet is stable with -20dBm reading.

Bear in mind that TM plays dirty with Maxis customers on HSBB. If you are on Unifi, you are going to get a dedicated port with -16dBm reading which is super good. And if you are on Maxis, your main port would be splitted into two and there is some degradation in term of optical power.