Hello @Basti099 ,

Welcome to our community.

According to Google Translate, your question is that you failed to log in to the extender. If there is any discrepancy, please describe it in English.

Here are some steps you can try:
1. Forget all saved Wi-Fi networks on your phone, then reconnect specifically to the extender's Wi-Fi network. Once connected, try opening the Tether app again. For detailed instructions, please visit How to remove existing wireless network profile on Android&iPhone.
2. Try logging in using another smartphone or tablet.
3. Power cycle the extender by unplugging it from the power outlet for about 30 seconds. Plug it back in and wait for it to fully restart.
4. Reset the extender to factory settings. During setup, make sure to configure the extender with a Wi-Fi name (SSID) that is different from your main router to avoid any conflicts.