RIng Doorbell 2nd not connecting

Hi I have a ring doorbell 2nd generation but it wont work. It always failed with „setup did not complete check your internet connection“. I already got a second device but the failure is always the same. I also checked different setups like: -) Different Passworts and channels on the LTE Router (TPLINK) -) Different Smartphones for the configuration (Huawei and Samsung) -) Multiple factory resets -) Connection via a TP Link WLAN Extender But nothing helps and I don’t understand that because all other devices have no Problem with the internet and Wlan. So is there any special hack that I can do or is the ring doorbell an unusable product.