I have bought and registered my O2 number in Germany in December 2022, but, I have been outside of Germany for the last 5 months.
I got an email saying that I should recharge my account before 17th of May 2023, or it will be suspended.

I am unable to recharge my account through the O2 portal, since it only accepts European banks (and Serbia seems to be in Antarctica).

What recharge system/website can I use to do this?

I will be back to Germany in the beginning of June this year.

o2_Giulia, 11.05.2023, 15:46, moved from Austausch rund um die O₂ Community to English O₂ Community: Mobile