you can change from any telco to any other telco that you like while maintaining your number.
Just go to a new telco, tell them you want to subscribe but using your same phone number.
They'll know what to do. For your research purpose, you can google "MNP" or "Mobile number portability"

Before that, try changing ur APN settings to "redone" or "celcom4g". See if there's any improvements.

If you dont have any contract, you just talk to the other telco only.. no need to go back to redone.
If you have a contract, I'm not sure. I think, it should be the same also. No need to go back
to redone. By the one of the month, redone will simply sent you a bill as usual, but this time it will include a
penalty.. Anyway, they will advise you again when you go to them ..