I was talking to one of the tech support today, but she doesn't have any idea what I was talking about and suggests me to talk to SE Support. They keep claiming their internet is "Fine" and I mentioned people who plays this game under the same ISP were having the same issue, again, unhelpful response. Better try my luck contacting through other ways.

I never had any issue with this game until recently, but since not a lot of other players are affected (at least of some aussies with different ISP), I'm definitely think there's some problem from Optus' side.