Before I ever forget it again and then have to call the info hotline again, here is the procedure for how to use a router e.g. Netgear or Arcor to a telecom connection. The problem is that T-Online has 3 identifiers or user name, but a normal router understands only one user name. You have to write the first 2 identifiers (the long numbers) one after the other, then make a "A", the short identifier e.g. 0001. Then you have to attach the t-online.de" to the " Thus, one turns the 3 identifiers into a user name, which Telekom also understands and which can also be entered into commercially available routers. The password remains the normal password, as it is also stated in the letter from Telekom.
Maybe this will save some of you some time.