Switch to a "Big Talker" tariff. This gives you some allowances each time you top up but it also allows you to retain your credit to use as a normal PAYG phone.
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)