This is likely an issue with your location not receiving optimal cell coverage. However, you should also check to make sure that your mobile device isn't set to low-data mode as well.
The data from your regular plan will not carry over to the next month, so if you don't use them, they will just get reset the next month. However, data included as an Add-on will carry over to the next month until you've used up all of it. Note that,...
