Hi Patrick,

you are right, there is no rail connection from Tegel Airport to the City.

However, Schöneberg is not very far from TXL, so the journey should not be too bad.

Depending on where exactly you are staying, you can take either the local train (S-Bahn) oder subway (U-Bahn) when you get off the bus.

This map always helps me to get along when I am in Berlin, so maybe this will be helpful for you too:

https://www.s-bahn-berlin.de/pdf/VBB-Liniennetz.pdf

TXL is in the upper left section of the map.

Have a safe and fun trip!

Peter