the simplest way by far is train (mostly RE) to zoo station but it’s a bit more than 10 minutes walk from there and that’s if you go for the fastest walking route which is through the Tiergarden so if you are dragging luggage it might not be ideal and the walk which stays on pavements instead would be about 20 minutes.

There is no closer/easier u bang station, the closest public transport is a bus which is at the nearest bridge so you could technically take a bus from a station to that bus stop but between the wait, change etc I personally really don’t think it would be worth it.