Is your data usage already counting as usual or still at max(usused)?
Coz the extra 500MB (each month in 2 months) will use first. and the monthly data usage still max until you use up the 500MB.
I guess better contact CS since already 24th, both of my line auto started at the morning, first day of this billing cycle with a message reminds me.
May be you can try Twitter DM(last time took me 6 hours for a reply but I guess should be much faster now) if you don't want to wait on hold.