Hi @ShengBerlin 

Welcome to our o2 Community. 

Our o2 my service app can support you with connection problems
You can download the app for your smartphone for free here (you can switch to english language after installation)
Then you can use the app to check your DSL connection for errors.
If the app cannot fix the error, it generates a process number,
with which you can then report to the “DSL Störungs hotline“ (in case of a mailfunction),
where the colleagues will then take an error ticket with you to check your connection (the app will show you the number)

When calling our customer support Hotline please remember that it is possible that you wont get a colleage on the phone who speaks english,

so if you are not comfortable or able to speak german, please ask a friend to call in your name.

Best Matze