Worst support experience so far: German ISP sent their cheap default router which I opted for, hoping that I didn't need to rent a costly FitzBox. Provider activates the connection, everything fine, but slow and unstable on my Linux laptop only. Try using their website, their support chat etc. as they made it very hard to even open a support ticket. I gather all the information, ping, traceroute, netstat, logfiles, router settings, broadband measurement etc. and finally manage to open a support ticket of my issue adn they say they will send an engineer to my place in about 4 days. I stay at home that day and wait for the whole day, nothing happens!
I get back to them, even get a phone call after waiting about 1h in the waiting queue, only to listen to someone tell me that the appointment had been cancelled as the ticket had already been closed. WTF! They said they measured my connection and could not reproduce any error. Did not even mention my lengthy log files etc. It took my own research and another hint that there is an incompatibility with that specific router and some specific devices so I should really replace their cheap router with a FritzBox device. What they also did not tell, that you dont need to rent their branded FritzBox, there are cheap second hand models from another ISP that you can reconfigure by simply chosing another ISP in a dropdown list that contains every popular German ISP. But why are they popular? You can only choose between the different bad ones, that's why they don't seem to make an effort.