It's too difficult to report an issue to the customer service.
1. TOBi only communicates in German. I tried to diagnose the issue via "Störung finden" function but it concludes the internet works properly after diagnose, which is a bull*piep*!
2. Finally I got a way to request a call-back service after choosing "miscellaneous" option to answer question "What best describes the disorder". Then I got a call back. While I use google translator to explain my issue in German, the hotline service agent understood it finally, but he tried to talk with me in German. After knowing I only speak English, he politely explained below words in English. "Sorry, I can only speak German", "You should find a person who can support you for translation in German, and call this number again". What a shame! He gives me a perceptation that he can actually speak in English but he doesn't want to do so. Similar case happened before during last time of issue. I simply don't understand why!!!
So the situation is, with support from ChatGPT, I can almost concluded the issue is not from my side, i.e., I didn't change any configuration, the DSL cable was connected properly. The actual issue is because they can no longer allowcate an IPv4 address to my address since Thursday (IPv6 address can be allocated after repeatedly retry), something must be wrong from their side, either due to an upgrade event or resource allocation problems. Without their support I can do nothing.
Too bad experience! I won't be hesitate if there is a chance for me to terminate my contract before expiry date.
