Hi @아크말, perhaps what you can do is to report again to customer service so they know this is not your phone setting issues and they should be able to give you a better solution.
The internet has become slow. It began in the last Friday.
yes same here..it happened last Friday until today...unstable internet line and super slow. Obviously celcom is having some problem but didn't even want to inform their customer, leaving them to complain and complain.