A little bit of update, customer support called me twice already, first was on Tuesday, they told me that their network has no problem everything working well, I told them I know your network has no problem, question is why I am getting with only 5-6gb speed while if I try different Celcom sim(different plan), I can get 18mbps, is my bandwidth capped? They OK OK and will feedback to tech team.

Yesterday, they called again and replied me the same answer, "No problem with the network", I was absolutely furious, it's just a meaningless answer to me. I demanded to speak to tech support, I told them you are not understanding the problem but they said they can't patch me through tech support unless I call them myself.

I told them "please tell tech team to call me tomorrow, my question is simple, i get slow speed, slower than expected, why? Is my line capped?" Till now they haven't called me back.

EDIT:

Okay I just gave them a call, this time they said still investigating, I ask them to give me a timeline, he said don't know, I tell him to ask the guy responsible and tomorrow let me know when. I told them that I will not let Celcom sit on this and hope it goes away, I also told them that even when I buy a house, they know roughly what time to hang over the house to me, tak kan this investigation has no timeline?