I3LiTZ

Update:

I got the game working properly and already played a match. The DMZ thing seems to work!

The story, so that I don't look like an idiot: My router is ten years old and the imprinted descriptions (what each button will do) is not visible anymore. The button that I had pressed restarts the router, but it's not the reset button. The reset button is a super tiny hole at the back that I had to poke with a needle. My router was reset successfully and I could access it, but I couldn't log into the network / internet again. It turned out that my login data was incorrect, because -- without telling me -- my internet service provider changed my login username from I3LiTZ@blabla-tel to I3LiTZ@blabla-net only because they changed their company name from shitty-ISP-tel to shitty-ISP-net beforehand.

Thank you, Viper, for trying to help me, even though it didn't matter after all.