Incorrect support for DSL Problem

Hello I had O2 Internet working well for the last 2 months without any problem. Last Friday someone fRom O2 or Telekom came to configure my neighbours Internet as he requested the service also to O2 (we live in the same building but entirely different apartments). Since that moment we do not have Internet  (Dsl/power light of the fritzz box intermittent). Supposedly a Telekom person was coming today to fix the issue but nobody came. I have been the last hour on hold on the o2 hot line. Nobody takes responsibility for an issue that was caused not by me.

How I can get the proper attention and solution as it's supposed To??

Thanks

Matias