Mobile data does not work

I am currently in the UK and my mobile data does not work. My phone is connected to the mobile network, but it just will not connect to any kind of mobile data (2G, 3G, 4G). It was working normally on the first few weeks, but after 2 months, it just suddenly stopped. I already went to a o2 shop nearby but they said that they cannot help as o2 UK and o2 DE are not the same.

How do I fix this? It is very inconvenient to not be able to use 3G/4G.