I have a similar problem. The data usage notification is not working and blocks the internet connectivity even if there is still data available in the data cap.
The pop up shows up, but does not load. It does not show if the allowance has been used up, or it's the warning that it's nearly used up. Because I don't know this, and the connection is blocked, I am forced to buy a data booster. I later on find out that I still have 20 GB of data available from 100 GB, so my data booster was totally unnecessary.
Please make this pop up warning work, or stop it from showing up and blocking me from using the data allowance I still have.