Yes, you can normally use Roaming which is for free - BUT: If you come across a certail treshold, you'll be charged more! There's a formula: https://www.o2online.de/service/auslandsoptionen/eu-verordnung/
Monthly price / current wholesale price * 2 = your usable GBs
wholesale prices are:
- from 01.01.2025: 1,547 €
- from 01.01.2026: 1,309 €