Customer services need to fix it for you, but in the meantime, you can reset your phones data counter each month to keep a track on your data, or download an app such as 3g Watchdog etc.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)