Hi @yvesv, welcome to the community,
After having consumed all the data allowed on your account (250MB in your case), you will surf at a reduced speed even if you are in place where there is 4G available.
As you said, it is ''up to 128kbit/s". It is not sure at 100% that you will hav that speed.
It is normal that you cannot download any application on the Play Store or try to go Facebook or whatever. This internet speed is very very slow. Even 2G is more faster than that.
This reduced speed is only here to let you check your mails or text message on What’s app for instance.
I hope my informations were helpful to you.