Trains to the city depart from the regional train station, not the long-distance ("ICE") train station mentioned in the thread title.

From T2 arrivals, walk outside to the street to the transfer bus stop (free shuttle bus between T1 and T2). Including wait as well as travel time, it should take around 15 min to get to T1. From the bus stop, if you look towards the Sheraton hotel (not towards T1), there are entrances to the terminal's lower level (you can see escalators/moving staircases behind window panes). Take one of the moving staircases to go one level down. From there, you'll see signs towards the regional train station. It's about a 3 min walk from the bus stop.

So yeah, in total it may take 20 min (slightly less if you get lucky and catch a bus without wait time).