Long story short, I bought my monitor more than 2 years ago from a small shop. In the first week of using it, the power supply stopped working, so I went back to the shop and they gave me a generic (not Asus certified one) as a replacement.
Went back home to test it out, and it worked. Since then, every month or so my monitor would not wake up after starting my PC, I had to either remove the DisplayPort from the monitor (not from PC as it didnt work) and wait around 10seconds and re-plugging it to open up again. Or unplug it from the power, wait 10secs then re-plug it.
I actually never thought it would be from power supply, and never thought of actually taking it back to the shop as these occurrences were rare (once a month, or once every 2months).
last year, it started to also flicker, like while gaming, the monitor goes dark for a split second then opens up.
I thought my monitor is losing its years of use. And I may had to buy a new one. But a quick search showed that these types of problems are occurring from power supply.