Turns out they're sending me a new modem. They say my Speedstream 5100 I received 11 years ago is outdated.
I'm just not sure whether that is the truth or just another trick to avoid doing any work on their part. Hopefully it won't be a Fritzbox or any other shiny new stuff that doesn't let you do any ajustments, phoning home once per day for failing updates, liking to contact the NSA and being the #1 target of the average scriptkiddie.