If there is nothing wrong with the phone itself, it should be a network problem.

I've been in this situation.

If there is a high demand for online experience, I suggest changing the network operator to telecom or Unicom (Northern Unicom, Southern Telecom). Great Wall, Aipu, Tietong and other famous brand networks are cheap, which makes sense, because they have limited resources and can't connect well when visiting some web pages or applications!